Removed useless Segment class

It was used solely for Segment.get (the only place where it was used just saved 
its .get() method, throwing away instance itself) and looks more like abusal of 
the purpose of the classes
Added ability to specify some segment keys once in top theme
Add ability to override configuration options

TODO: support for non-string scalars in vim overrides, ipython overrides, move
every non-ASCII symbol from every theme to the area (segment_data key) where it
can be overridden, tests.

Fixes #231
Replaced getfsize with os.stat