Merge develop into master.

+ Version bump to 0.4.2
+ Update requests requirement
+ Add sh requirement
+ Refactor main class

    Previous calls were like : `public_repos = bitbucket.public_repos()`.
    Now you would use : `public_repos = bitbucket.repository.public()`.

+ Refactor tests

    You can now run public tests with : `python -m bitbucket.tests.public`,
    And private tests with : `python -m bitbucket.tests.private`, after setting up your USERNAME and PASSWORD in `bitbucket/tests/private/settings.py`.

+ Add issues and issues comments methods (thanks to [davidmpaz](https://github.com/davidmpaz))