Merge support for GHC 7.8 and Cabal

Presently this fork will not build on 7.8.3.

schell has a 7.8 branch merged with cabal support from maximkulkin (modules, sandboxes). 

This PR merges your findsymbol patch into that branch. I thought I'd leave this here in case you have other vim-hsimport users wanting to upgrade their project to GHC 7.8.