Make the merginal buffer window have fixed size

It's kind of annoying when the merginal buffer get's bigger because you
close another buffer...
Fix #1 - Set nonumber in merginal buffer
Add the `mf` keymap to run Fugitive's `:Gmerge`

`M` and `mm` will still be used for Merginal's merge, which was created
before Fugitive's `:Gmerge` and can still be more comfortable for some
users.
Add `merginal#branchDetails`

Also make the functions use it - so now they work with remote branches!
Add ct and cT for tracking remote branches
Add ps,pl and pf to push,pull and fetch branches
Make dd be able to delete remote branches
Close #2 - add branch diff functionality

bd in the branch list opens the diff files buffer to diff against the
selected branch.
Fix the current-buffer checks the functions do

Instead of checking that b:merginal_repo exists, check the buffer name
to make sure the user is in the correct type Merginal buffer and not
only that they are in a Merginal buffer.
Pre-release actions

 - Bumped a minor version
 - Updated the README with the new commands