De-tabify all Haskell source files.

This library is a GHC core library and GHC is now built using the
-fwarn-tabs flag by default. De-tabifying this brings it into line
with GHC standard practice.

Also add -fwarn-tabs to the cabal file.

Closes: https://github.com/haskell/time/issues/18
Merge pull request #19 from erikd/master

De-tabify all Haskell source files.