Bug 1103121 - Add |mach gradle| command for building Fennec Gradle. r=gps

I went with gradle instead of gradlew because it's more likely to be
what users consider.  And mach helpfully fixes up the uncommon typo.

This is a little hard-coded right now but I don't think it's likely
any other Gradle consumer will arise in the short term.

--HG--
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extra : amend_source : d10846e845deda5d368bdfdbb5b3d68706038992
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