Added support for mercurial VCS

[pokoli]

Use .hg in the end of the repo to indicate that is a mercurial repo.

- Added mercurial for test dependency.
- Added test for mercurial checkout.
- Refactored vcs.py to reuse delete code.

[coveralls]

Coverage remained the same when pulling e672b86 on pokoli:mercurial_support into cf1fb09 on audreyr:master.

[coveralls]

Coverage decreased (-4.21%) when pulling 47fd5e1 on pokoli:mercurial_support into cf1fb09 on audreyr:master.

[coveralls]

Coverage decreased (-1.6%) when pulling de1cd76 on pokoli:mercurial_support into cf1fb09 on audreyr:master.

[audreyfeldroy]

Cool! Will review shortly. Thanks @pokoli!

[audreyfeldroy]

Merged into master!

Heads up that I made a big change:

1. Users no longer have to append .hg to the end of the repo URL.
2. And in fact, if you append .hg, Cookiecutter won't recognize it, since it's not a valid clone-able URL.

I decided this based on the fact that hg clone https://bitbucket.org/audreyr/cookiecutter-bitbucket works but hg clone https://bitbucket.org/audreyr/cookiecutter-bitbucket.hg doesn't work.

Thank you @pokoli, appreciate your work here. I will submit a pull request to cookiecutter-trytonmodule to reflect the above.

I should note that at the moment, with the changes in this branch Cookiecutter will only work with Bitbucket and not general Mercurial repos. That is my fault, and consider that problem temporary. Hopefully we'll be able to replace the vcs.identify_repo() function with the equivalent from a package that @pydanny is working on, for identifying repos for OpenComparison/DjangoPackages.