Add a "set terminal title" segment.

Emmits a non-printing sequence that sets the terminal window title to user@host:dir
(This is a common default setting in many linux distros)

It was necessary to slightly refactor Powerline methods to distinguish between None and '' passed as parameters.
Fix use of escape characters in "set terminal title" segment.

Escape characters were incorrect for non-BASH shells.
Merge pull request #155 from paol/master

Add a "set terminal title" segment.