Bug 904105- part 2: create firefox extension

create a firefox extension for building gaia. you can follow instructions to use this extension:

1. cd <GAIA_DIR>/tools/extensions/gaia-build@gaiamobile.org
2. FIREFOX="<FIREFOX_BIN_PATH>" make run
3. go to tools -> addons and enable gaia-build extension (default is disable)
4. go to url: about:gaia
5. select your gaia dir and click launch gaia

another firefox window will be opened and running gaia.
Merge pull request #12416 from yurenju/xpcshell3

Bug 904105- part 2: create firefox extension r=@timdream