simplified download using the maybe function
Used a monad transformer for the downloaders

This simplifies the implementation of the downloaders, all they
need to care about is returning a value or failing; the
transformer handles the rest
replaced an if with a guard call
Merge remote-tracking branch 'origin/develop' into feature/downloaders

Conflicts:
	src/DepTool.hs
	src/Downloaders.hs
	src/Downloaders/Git.hs
	src/Downloaders/Local.hs
Merge pull request #6 from ericmoritz/feature/downloaders

Feature/downloaders