refactored segments, power line is now the class

Moved all of the independent functions into the Powerline class. Changed
segment so it holds just the information about each segment. Added bold
spport. Changed how colors are stored. Removed check for .svn folders
(not relevant with svn 1.7+).

The reason of most of these changes are:
 1. to support more foreground/background combos. That is, before the
 separators were hard coded.
 2. to lay groundwork for support for color config files
 3. to group elements logically: color info isnt scattered everywhere
 now.
fixed checking svn repo

As of svn 1.7+, there is no .svn dir in each folder. Changed
add_svn_segment to use svn info to check if the current folder is in an
svn repo.
cleaned up some old code
typo
changed the color map

Better for importing a config file
Added config files for the colors.
Added config file option.
Update README.md

List more features, clean up install info.
Update README.md

added note about colors
cleaned up options
Update README.md

Added option information.
Merge branch 'new_segments' of github.com:bradjc/powerline-bash into new_segments