Channelpack

* The persistent condition strings are written whith python syntax operating on
numpy arrays. Identifiers use replacement syntax like ``%(<id>)``.

* Removed method ``ch`` from the ChannelPack, superfluous.

* Added ``counter`` method to the ChannelPack.

* Added ``parts`` method to the ChannelPack.

* Change name on method set_samplerate, was set_sample_rate.

* A number of bugfixes.

* Two xldate conversion functions in pullxl.

* Important bugfix in pulldbf module. Was forcing types on numpy, biggest float
was 'f4'. In case of excel kind of dates for example, this is not enough, and
numbers were lost. The result was repeating equal numbers that should not be
equal.

* Since forcing types on numpy was deprecated, an issue with missing values in a
dbf file (nulls) was easy to fix. Now, null values are replaced with numpy
nan, was (0). Also very important.

* xlrd added to install_requires list in setup.py

* Docstring fix in slicelist method. Was wrong.

* Support for reading spreadsheet data (xlrd as backend).

* Add method slicelist to channelpack.

* Bugfix in function rebase in the ChannelPack class. Was a KeyError.

* Corrections in the changes file, (this file). Some changes was stated as
coming but already implemented, (assumingly with 0.1.2).

* Add some coming changes in this file.

* This version has no changes in code and no distribution. But docs are updated.