Get rid of last vestiges of profiles.

Auditors: pl
Fix a couple of code review nits.
Fix a couple of tests.

Also gets rid of some cruft.

Auditors: pl
Remove an unneeded (and confusingly named) ctor param.

MockTarget has been a subclass of Target for a long time, so this
trick is no longer necessary.

Auditors: pl
More verbose python chroot debug logging.
Add missing import.
Make source root target type specification optional.
Simplify source root registration.

Auditors: pl
Fix bug in source root registration.
Get rid of confusing @property stuff in TargetWithSources.
Small fixes in doc.py.
Get rid of spurious "skipped test" message.
- Fixed a bug where the Pants build process would fail to install distribute binaries into the virtualenv.
- Removed (presumably) internal twitter servers from the setup script.
Merge pull request #189 from teddziuba/issue-181

Upgrade virtualenv, fix issue 181
New custom zinc/sbt version.
Fix some double quotes -> single quotes.
Move some common stuff from {Java|Scala}Compile into JvmCompile.