add findgtest

use 
set(GTEST_HINTS path/to/gtest)
to indicate where to find gtest.
Although this is no a very good solution since you can install gtest anywhere you want, that means you have to supply GTEST_HINTS with find_pacakge(gtest), which doesn't differ too much from hardcoding it in cmake.
I think how ros does it is it find's the source file of gtest and compile it everytime.
Maybe we could both install gtest to /opt/gtest-1.7.0, so that we don't have to change the cmakelist everytime we pull from origin?