MDLSITE-3252 check for third party library modifications

This check will try and find modification to third party libraries
and warn about them if the thirdpartylibs.xml or readme_moodle.txt has
not been updated.

To test you can use the branch:
`git://github.com/danpoltawski/moodle.git thirdparty-test1`

Some things you can note in testing:
https://github.com/danpoltawski/moodle/commit/eada3650769da60d09d31cb5a233662e4f0ad7fd is detected as modification to addodb
https://github.com/danpoltawski/moodle/commit/3eaf3c1a158014f0406ae6d00b474b9c4dff90a8 is detected as modification to rangy
https://github.com/danpoltawski/moodle/commit/29a6bab8379c8363e0c4329e670efb26a03f6119 is detected but not warned about because of https://github.com/danpoltawski/moodle/commit/7eafb8a0b4e2dd92a5687c25edf64e75b3bb761e (confirm by reading the thirdparty.txt) 
https://github.com/danpoltawski/moodle/commit/0e1a1c2a01c01f74f7f795485c4b85ec4b5877cc is detected but the message doesn't mention readme_moodle.txt, but instead the existing moodle_readme.txt

Sidenote: I think these things could become an error at the point when the readme_moodle.txt file was always known. (Not the case here, we guess)