Add groupMap and groupMapReduce extensions

[NthPortal]

Fixes #11

[NthPortal]

I think there's some MiMa twiddling that needs to happen, but I believe this code should pretty much work

[julienrf]

Unfortunately, this PR is not backward compatible (see https://stackoverflow.com/a/18369508/561721 for an explanation of why).

I think we could create a 2.x branch where we could apply backward-compatible changes (if needed).

And branch master could be the place where we prepare the 3.x release…

What do others think?

[lrytz]

Maybe the toTraversableLikeExtensionMethods method can be duplicated and added directly to the compat objects, instead of the PackageShared super trait? That would avoid the binary incompatibility on 2.11.

[NthPortal]

for future versions, can PackageShared be an abstract class rather than a trait (thus sidestepping the problem)? I don't know the rules about what a package object can and can't extend

[NthPortal]

@lrytz yup, I'll do that

[julienrf]

@NthPortal Yes, that should work (see #292)

[NthPortal]

cleaned up, squashed, and ready for merge