Asammdf

There were a lot of fixes since release 2.8.1 and development for 3.0.0 will still take some time I've decide to make this bugfix release.

Many thanks to all who have contributed to this release!

Changes
- continuous integration using Travis CI
- unified code to handle mdf versions 2 and 3
- option to get raw channel data using the _get_ method
- asserts to validate proper block ID read from files
- allow passing MDF instances to the merge method, alongside string file names
- specify channels by name or by (name, group index, channel index) in the filter and select methods
- added __iter__, __len__, __contains__ and whereis methods to MDF class
- try to use chardet if installed in case of incorrect string encoding


Fixes
- Use numpy.where for signed non-byte-sized integers
- fix rational conversion bug in mdf versions 2 and 3
- fix error in saving mdf version 4 RTABX channels
- fix saving of channel composition channels
- fix get error for generated MDF files (result of merge, filter, cut)
- fix error when filtering contains a channel composition and memory=='minimum'
- fix for python 2 unsorted file
- fix compacting of integers on save
- fix save methods
- mdf version 2 and 3 compliance fixes
- fix appending of text conversion with empty strings in mdf version 4
- fix mdf version 4 text block when source string is empty
- fix error in mat export
- fix error when appending signals with big endian byte order
- fix error in case of invalidation bytes
- fix handling of file names in save method

Changes
- continuous integration with Travis CI (thank to Mihai)
- initial test (thank to Mihai)
- avoid unnecessary duplication of channels when filtering measurements with channel compositions

Fixes
- fixes for channel composition in mdf version 4 (thanks to Julien Grawe)
- multiple documentation fixes (thanks to Mihai)
- multiple fixes for mdf version 2 files

Changes
- support MDF version 2.14
- add new _resample_ method to _MDF_
- try to avoid _MemoryError_ for _append_ on 32 bit Python
- add _get_channel_comment_ and _get_channel_unit_ methods
- add save overwrite as configurable option
- added possibility to return pandas DataFrame to select method
- pandas is now a required dependency


Fixes
- use data block size equal to multiple of record size when splitting data blocks in mdf version 4 save
- fix default values for memory argument in MDF methods
- fix incompatible plot title in Signal
- handle backslash in channel names
- fix typo in usage example
- fix _iter_to_pandas_ error
- fix and improve _info_ methods

Fixes
- fix missing attribute error in _Signal_ class when plotting surface (map) signals

API change
- _load_measure_data_ argument was replaced by _memory_ argument:
- _load_measured_data=True_ ⇔ _memory="full"_
- _load_measured_data=False_ ⇔ _memory="low"_
- _memory="minimum"_ is new and will use the least possible amount of RAM (see http://asammdf.readthedocs.io/en/latest/benchmarks.html for benchmark results)

Fixes
- fix errors in _save_ method
- fix errors in _merge_ method
- fix error in _cut_ method for channel groups with no cycles
- fix mdf 4 _TextBlock_ 8byte alignment
- fix wrong link to sample reduction block in _ChannelGroup_ for mdf 4
- fix append and get in case of mdf 4 TTAB conversions
- fix error in case of mdf 4 TTRANS conversion

Fixes
- fix error in _MDF3_ _save_ method
- fix _select_ method to return the signals in the same order as the channels given
- fix _get_ method when channel group cycles number is 0