Added a pep compliant version numbering scheme

Hello,

This is the first of a few PR's I'll be doing on this - this one simply puts a PEP compliant version number approach.  

* put the version into `sslify.__init__` as a tuple
* added a simple function to construct version number from tuple
* assigned output of function to `__version__` variable
* updated `setup.py` to use the variable containing the version rather than have it hardcoded in two places.

Nice work.

Cheers, 

Jamie.