Improve support for out of source builds, fix urdf linking

There are many places where temporary build files are installed
into the source tree ${CMAKE_SOURCE_DIR}, when they should be
installed to the build folder ${CMAKE_BINARY_DIR}. Also, I was
having trouble finding the urdf libraries, so I fixed
FindDARTExt.cmake.in
Remove src/dynamics/CMakeCache.txt (issue #77)
Merge pull request #78 from dartsim/out_of_source_builds

Improve support for out of source builds