test: Bundle GTest/GMock 1.8.1 sources and provide a find script

GTest/GMock is a unit testing framework that we want to use for
writing unit tests for Quassel in the future. Since Google
recommends building the test libraries with the same flags and
settings as the code under test, we should build them as part of
Quassel's build process rather than relying on system-provided
libraries.

Because most distros don't seem to package the GTest/GMock sources,
bundle them with Quassel. The upstream sources are unmodified,
although the set of files shipped is trimmed down significantly
in order to not needlessly blow up the repo size.

Provide FindGTest.cmake, which builds the libraries from the
bundled sources.