Remove unimplemented config vars
Use default optimization level in Cabal config
Use default options when bootstrapping Cabal
Default to not removing any installed directories
Support Cabal 1.20.0.3
Support GHC 7.8.3
Remove unhappy workaround
Support building customized sandboxes
Improve message wording
Rename prepared to prebuilt
Improve notation
Improve sandbox validation and failure logic
Generalize sandboxed_cabal_do
Remove extraneous expectation
Add customize script helper and example
Allow omitting app version when faking build