Fix a typo in pex.py which results in NameError. s/extra_paths/extras_path/
Merge pull request #149 from orsenthil/master

Fix a typo in pex.py
Missing imports
Fix bug in exclusives.

- IvyResolve was erroneously using the wrong key for a target group with no declared
  exclusives.

Auditors: benjy
Remove debugging print. (Sorry about that)

Auditors: benjy
Exclusives bugfix.

Builds with no exclusives would sometimes fail due to a bug
in how execution order was planned.

Auditors: benjy
Remove old classpath code.

- Now that all code is using the new exclusives-based classpath
  mechanism, get rid of the old one.

Auditors: benjy
Tweak exclusives error message.

Also tweak goal name.
Merge remote-tracking branch 'twitter/master'