Update .gitignore

Ignore auto-generated files that result from
"android update lib-project" (which is invoked within "ant debug").

Some of the plugins (library projects) already have their own
.gitignore file which includes the exclusions made here.  This
commit assures that the exclusions are applied to *all* plugins,
regardless.

This, of course, has no effect on files that have already been
intentionally checked into the git repository (such as the main
build.xml).