Ensure that the lib/themes directory exists before installing to there

(An alternative would have been to add an empty lib/themes/.gitkeep
file to this directory, but that makes ignoring the installed themes
more complicated.)
Precompile assets only for production.

Conflicts:
	script/rails-post-deploy
Try to uninstall the old theme from vendor/plugins and lib/themes

The theme install task would fail if there's an old theme present
in vendor/plugins, since it doesn't try to uninstall the plugin from
that location, only the new location.  Then when the install.rb
in the new plugin runs, it'll complain that there's a
public/alavetelitheme symlink already present.

This commit changes themes:install to try to uninstall the plugin
from both locations.
Merge branch 'theme-install-fixes' into rails-3-develop

Conflicts:
	script/rails-post-deploy