Make it possible to override source URL

To make test builds it can be useful to point gitian at an
alternative repository.

This commit makes this possible by adding a `--url PAIRS` command line
option, that allows to optionally specify where to fetch the code, for
example:

    gbuild --url bitcoin=https://github.com/laanwj/bitcoin.git --commit bitcoin=2014_03_...
Merge pull request #51 from laanwj/master

Make it possible to override source URL