Simplify tradeoff between accuracy and speed.

If the plugin slows down your Vim too much, you can have it run less
often by setting `g:gitgutter_eager = 0`.

This replaces the former options `g:gitgutter_on_bufenter` and
`g:gitgutter_on_focusgained`.
Clarify that plugin works with saved buffers.
Clarify FAQ.
Add default mappings.
Merge branch 'master' of https://github.com/airblade/vim-gitgutter

Conflicts:
	plugin/gitgutter.vim