Bumping `base` lower bounds to 4.7.
Migrate from `type-level` lib to `GHC.TypeLits`.
Updated haddock documentation to reflect TypeLits usage.
Bumping version to v0.2.0
Updated Travis-CI configuration to us GHC 7.8.
Merge pull request #8 from aloiscochard/feature/typelits

TypeNats support