Channelpack

* Bugfix of function _slicelist() in pack.py:Channelpack being called
every time the pack was called on a part. A chached slicelist is
used now instead and updated only when the mask is set anew.

* A Makefile fix in linter rule.

* Better handling of missing values in text files.

* Bugfix to respect encoding using the contextopen function in
readtext, d1e26a.

* readtext.py: Don't blindly strip all white space from lines (made
converters argument have no effect).

* The project rewritten completely.

* Python 3 and 2 supported.

* History prior to this release might be erased.

* Allow open file objects to be supplied to the ``txtpack`` function,
not only the file name as a string.

* Some pep8 work done in the core python files.

* Bugfix in pullxl.py. An update in xlrd version 1.0.0 on empty cell values made
this bug evident. Empty cells was reported as empty bytestrings before, now it
is reported as empty unicode strings as documented. The channelpack bug
resulted in all values in a "channel" being unicode strings when some should
be numpy.nan and the rest numbers.

* Added a records method to ChannelPack.