Install/uninstall commits in env

[Chris Warburton]

Allow different commits to be "installed"/"uninstalled" to an
environment.

The environment contains the dependencies (as specified in 'matrix' and
implemented in 'builders'). The project being benchmarked can be
"installed" from a particular commit.

asv assumes that the project is a Python package, and hence the
benchmark scripts just need to import them. We might as well do the
same.