Changes to implement real marks in vim-mode.
Cleaning up the commit and adding the specs.

For real (tracking) marks in vim-mode.
Using the Marker api to implement the vim marks.

Now we don't store the location we store the head of a buffer Marker from the
atom API. This tracks the changes as needed.
Merge pull request #281 from carlosdcastillo/real-marks-pr

Real Marks