Update default config to match current behavior.

Having the extra prefix is confusing to people.

Refs #215
Invalidate build cache including behavior to prevent timestamp being cached before build is compiled.
Merge branch 'master' of github.com:bigfishtv/asset_compress
Check for a valid timestamp before finalizing.
Add tests for invalidate and finalize methods.
Merge pull request #217 from bigfishtv/master

Don't overwrite cache timestamp until after build has been compiled and saved.
Bump version number.
Merge branch 'master' into 3.0