Added buildtools as a subrepository
Simplified build script now that buildtools are guaranteed to exist
Marked compatible with Firefox 19.0a1 & Co.
Marked compatible with Firefox 20.0a1
Added .hgignore file
Removed branchname metadata setting, no longer used
Marked compatible with Firefox 21 & Co.
Updated to current build tools version

--HG--
rename : metadata => metadata.gecko
Marked as compatible with Firefox 22
Updated compatibility range (at least Firefox 16 required now due to bug 726125)