Merge pull request #97 from bulwahn/rostest-depends-on-rostest-native-v2

rostest: rostest depends on rostest-native (resolves #83)
gtest: add gtest class

This adds a gtest class which injects gtest into the directory given
by GTEST_DESTDIR. The default value is S. This enables projects using
gtest without having it in the source repository.
tf: was missing gtest (resolves #95)

This fixes the missing gtest error in the do_configure step.

lb: reworded commit header, added reference to issue
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@oss.bmw-carit.de>
class-loader: was missing gtest (resolves #86)

The class-loader requires the gtest sources. This commit resolves
this issue using the gtest class.
README: Tobias Henkel is a contributor