pip install from directory containing requirements.txt

According to the [pip docs][1] (see example 4), pip/requirements.txt can
install relative paths. In order to do so, pip has to install
requirements.txt from the directory it's in.

[1]: http://www.pip-installer.org/en/latest/usage.html#examples
Merge pull request #13 from rouge8/master

Run pip install from directory containing requirements.txt