Made clean task read from the yaml for all the configurations.

Cleaning a non-existant target causes a failure (and AdHoc is custom).
To preserve backwards compatibility, AdHoc is still included by default
for previous thrust.yml that do not specify a configuration key.

Removed sdk when cleaning (to support cleaning OSX spec targets).
Switched to using xcodebuild -list to get all available build configurations to clean
Merge pull request #15 from jeffh/smarter-clean

Smarter clean task