Add Firefox support

The release.bash script has been updated, and now depends on a helper
script called generateManifest.bash. A "--browser" flag (shorthand "-b")
has been added to the script in order to specify the target browser.

Now the manifest.json file is saved as a template at
templates/manifest.gjson (gjson stands for Generic JSON), which includes
conditional clauses which indicate what parts of the manifest are
applicable to each browser.

generateManifest.bash generates the src/manifest.json file.

Change-Id: Ib051f3d893045c1377007b9b076ffa7a702d9648
diff --git a/.gitignore b/.gitignore
index ec335e0..ef9d424 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,2 @@