Clean source downloads from build cache

A big cache slows down subsequent builds
Allow external cache retrieval
Document using Anvil with EXTERNAL_CACHE
Merge pull request #45 from begriffs/external_cache

Apply Anvil cache to pushes
Detect failures and stop the deploy

avoids broken builds in production
Merge pull request #48 from begriffs/failfast

Detect failures and stop the deploy
Guard script against nonzero cabal configure return

It is harmless
Merge pull request #51 from begriffs/guard_cabal_config

Guard script against nonzero cabal configure code
Snap is not necessarily in the path, use cabal exec

Thanks to @totherme for pointing this out
Install brand new libgmp
Look in correct place for prebuilt libs
Expose ghc-includes to slug
Install prebuilt library to cache for its later restoration
Ghc 7.8 dist filename change
Install fancy libs before attempting to potentially build ghc
Docs and defaults for fresh versions
Merge pull request #53 from begriffs/ghc7.8

GHC 7.8, cabal-install 1.20.0.0
Keep happy, alex etc in ghc-utils
Add ghc-utils to PATH before trying to build app
Merge pull request #55 from begriffs/version-agnostic-utils

Version agnostic utils