Axon python3 support, new ABF1 protocol features, and bug fixes.

Files affected: axonio.py
* Adds python3 support for ABF1/2 file formats. One major change is that all
string data is now treated as bytes in python2 and python3.
* Fixes a significant bug in the parsing of ABF1 files related to the sampling
order of channels not being considered during block loading. ABF1 file parsing
now properly follows nADCSamplingSeq.
* Added new ABF1 header fields that make it possible to reconstruct the protocol
for a given ABF1 file. The source of information for these new header fields is
get_abf_header.m from: http://sourceforge.net/p/neurodata/neurodata/ci/default/tree/.
* Changed the clean_string method to make it faster.
* Tested on ABF1 and ABF2 files on python 2.7 and python 3.3.