Added .hgignore file

--HG--
extra : rebase_source : 70bb04f7e2a00557449576666afb0831daecbdd1
Marked compatible with Firefox 20.0a1

--HG--
extra : rebase_source : 080ddf036510d0ebe75f542bf3d7b2b1f3df2e3a
Removed branchname metadata setting, no longer used
Marked compatible with Firefox 21 & Co.
Updated locales from Crowdin
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)