Remove hg from default backend

This is imcompatible change.

Because almost of git-gutter users use only git and don't use
mercurial. To check that the file is controled by mercurial takes
time rather than git.

User should add hg to backends list if he/she wants to use git-gutter
for mercurial project.
Update README
Merge pull request #73 from syohex/change-default-backend

Remove hg from default backend