Idap gds merge

__* PR #31 was originally opened to merge GDS Verify changes in to this module. After some comments, I thought it best to close PR #31 and create a new branch with cherry picked changes. Hopefully this should be a bit clearer than one big commit (as is the case in the original PR) *__

GDS Verify(IDA) project use a fork of this graphite module. It seems like a good idea to incorporate changes in to this module, with the view the Verify project dropping the fork and using only this module.

The GDS Verify additional changes are:

Ability to specify the user used to run the processes
Install from Debian package
Some carbon cache options