Implemented: force python 2 or 3 with ability to switch during runtime

Simultaneous python2 and python3 support, switchable during runtime
calling a function as well as setting a variable in .vimrc! Closes
issues #200 and #210 (which gave the idea for this commit, thank you
@Shtucer).
Load only default/forced py_version, changed names and variable type

1. Changed pycmd: renamed to py_version, changed type, resulting in
...#force_py_version = <int> (2 or 3)
2. Changed structure of initialization, changes broke functionality
(<sfile> behaviour changes inside a function)
3. Cleaned up.
Merge pull request #232 from eyetracker/dev

Implemented: force python 2 or 3 with ability to switch during runtime