Implement per-compiler timestamps

Implemented by changing the type of timestamp data from `[AddSourceTimestamp]` to `[(String, [AddSourceTimestamp])]`, where the `String` parameter identifies the compiler & platform (e.g. `i386-linux-ghc-7.4.2`).

`cabal configure` now adds a timestamp record for the compiler we've switched to. An initial timestamp record is created by `sandbox add-source`.