Bug 859573 - Part 2: API for declaring virtualenv packages; r=aki

This patch provides a formal API for declaring virtualenv packages.
Before, scripts had to override create_virtualenv to install custom
packages. Now, they simply need to make a few API calls at setup time to
register packages and the default implementation of create_virtualenv
will do the right thing.
Bug 859573 - Part 3: Record system resource usage of scripts; r=aki
Backout e31f226cb264 for bustage
Bug 859573 - Use super() in DesktopUnittest.__init__; r=bustage
Bug 859573 - Part 3: Support optional Python package in virtualenv; r=aki
Bug 859573 - Part 4: Record system resource usage of scripts; r=aki
Bug 859573 - Trap ValueError encountered on B2G emu (vm) tests; r=bustage
Bug 859573 - Don't print original values when debugging exception; r=bustage

Printing the values confused the mozharness logging infrastructure and
threw an additional error.
bug 893574 - switch sendchange master. r=bustage
Backed out changeset f38888be196c (wrong patch for bug)
Merge Backout
Bug 887888 - Bug 829211 made Android Panda logs repeat every line twice. r=aki
Backed out changeset c8f22b42b51e
Bug 887888 - take 3 at properly landing: Bug 829211 made Android Panda logs repeat every line twice. r=aki
Bug 811409 - Add support for running c++ unit tests to desktop_unittest.py; r=aki
---
 scripts/desktop_unittest.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--HG--
extra : rebase_source : 3b2182dae61522b6cc89ca5509f3e0bda3c68ba2
bug 859573 - followup fixes. r=bustage
Added FENNEC_23_0b6_RELEASE FENNEC_23_0b6_BUILD1 tag(s) for changeset production. DONTBUILD CLOSED TREE a=release
Added FIREFOX_23_0b6_RELEASE FIREFOX_23_0b6_BUILD1 tag(s) for changeset production. DONTBUILD CLOSED TREE a=release
backing out 9abede48fb56 for bustage.
bug 894462: servo build slaves need libpng. r=aki