Respect Rails.public_path when computing the path to the manifest file.

This is important when packaging JRuby apps using warbler, as the
public directory configured to be the root of the war file, rather
than underneath a directory called "public".
Removed extra blank lines
Use the configuration of given application instance to get the public path rather than the Rails constant
Merge pull request #87 from swisener/master

Respect Rails.public_path when computing the path to the manifest file.

Conflicts:
	lib/sprockets/railtie.rb
	test/test_railtie.rb
Add CHANGELOG entry for #84