ghcjs buildable version

[buckie]

Big fan of the library, including the api upgrades from 0.3.5.5. I needed to build this via ghcjs and cpphs or thyme.h was blowing it up (linker errors, couldn't figure out which one). Regardless, this version built fine on OSX for ghcjs at the cost of the lens flag. Overall I dislike having a little custom lens as more and more people start using this lib but I do see the motivation as the build times increase. I think given stack and new-build that one time build hit is no longer so onerous.

[erikd]

@buckie Your PR doesn't build with ghc 7.6 of ghc 7.8:

src/Data/Thyme/Clock/TAI.hs:85:52:
    Not in scope: ‘mempty’

Versions of GHC earlier than 7.10 did not include Data.Monoid in the standard prelude. Fixing this probably involves adding something like:

#if __GLASGOW_HASKELL__ < 710
import Data.Monoid (mappend)
#endif