Git.clone() added

* Wrapper for `git clone` command.
* Test added and custom 30000 timeout is set to it as cloning requires
quite some time.
* Documentation updated.
Git.init() extended with bare parameter

* When bare parameter is set to true, bare repository is initialized.
* Tests added for bare and non-bare initialization.
* Documentaiton updated.
Disable Git.clone() test

This test fails on Travis as cloning from GitHub requires accepting the
host authenticity, expects user input and eventually fails on timeout.
It makes sense to uncomment this test only locally.
Clone using HTTPS to avoid authentication prompt
Merge pull request #28 from skovalyov/git-clone-and-init-bare

Git clone and init bare