Add StimfitIO, including unit tests, to neo.io

### Notes
* Requires the [latest python-stfio code](https://github.com/neurodroid/stimfit)
* Only tested with the [latest biosig code](http://biosig.sourceforge.net/) (may work with earlier versions though)
  
  ```
  git clone git://git.code.sf.net/p/biosig/code biosig-code
  ```
* Build the stfio module using
  
  ```
  ./configure --enable-module --with-biosig
  make
  sudo make install
  ```
* ~~Unit tests are currently "hijacking" AxonIO abf test files. Since these are not yet in the stimfit subdirectory on G-Node (https://portal.g-node.org/neo/axon/), they need to be manually copied into the temp folder.~~

### Issues
* datetime isn't producing a correct representation yet. We're working on it.
* ~~Tests take a long time compared to AxonIO.~~
* ~~t_start isn't set correctly; all segments have the same t_start (the total duration of the file). Not sure why this is the case; t_start is set correctly in l.114 of stimfitio.py, but it somehow gets overwritten~~
* Events are not currently converted.
* Only reads entire files.