cabal-install/IndexUtils : Expose function getIndexFileAge.

* The function has type 'Repo -> IO Double'.
* It returns the age in days as a Double.
* Places that expected the age as an Int have been updated.
* Builds with ghc 7.4.2 and 7.8.3.
cabal-install/IndexUtils : Fix display of age now that its days as a Double.
Merge pull request #2150 from erikd/master

cabal-install/IndexUtils : Expose function getIndexFileAge.
cabal-install: Add command 'cabal saved-config diff'.

This command prints out a psuedo-diff between the user's ~/.cabal/config
and what cabal would generate if it did not exist.