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
Revert "Replace on_load with instance_eval"

This reverts commit 04dfaf74eecca0745efa3e45636d76524380d6f6.

reason: Killing the lazy-load hook obviously causes performance regressions in some cases
Make sure that AV::Base is activated before running the Rake tasks
Restore `config.assets.manifest` option
Merge pull request #116 from amatsuda/lazy_load_action_view

Bring back action_view lazy loading
Merge pull request #122 from johnnyshields/restore-manifest

Restore `config.assets.manifest` option

Closes #107
Improve CHANGELOG
The default value should be nil
Test against Rails master
Test against 4.1.0
Remove support to Rails 3

It was never supported officially since it needs to use specific Rails
forks so we are removing support without a major bump.
Master is 2.1.0
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