MDLSITE-2003 - php syntax checker

To avoid syntax erors in the codebase. Note that this just runs lint
against the current state of the files in the codebase. We could do
something cleverer with git to checkout the contents of the file from
each commit, but in the end this way is simpler and doing less work and
checks the current state of the repo - so its probably no bad thing.

Also detect BOM characters (thanks to Matteo for the suggestion)
Merge pull request #24 from danpoltawski/MDLSITE-2003

MDLSITE-2003 php lint checker