Enabling passing SSH options to git commands

Prior this commit there was only very limited control one had
over SSH options in the `mcv.git` module.  This commit makes it so
that you can pass any SSH -o option (see `man ssh_config 5`) to the
`mcv.git` commands in the `ssh_opts` dictionary and get that SSH
behavior.

Example:

    import mcv.git
    mcv.git.clone(
        'git@github.com:framed-data/mcv.git',
        '/home/me/mcv',
        '/home/me/.ssh/id_rsa',
        {'StrictHostKeyChecking': 'no'})

Currently the args are all strings and need to exactly match what the
ssh shell command expects (e.g. 'yes', 'no')--there's no conversion
between Pythonic values like True/False to those strings.
Merge pull request #28 from elliot42/git-ssh-opts

Enabling passing SSH options to git commands