config.assets.manifest:
- fix Rake task
- fix config.assets.compile = false case
- add additional test cases related to config.assets.manifest option

Conflicts:
	test/test_railtie.rb
Update tests and README.md to reflect discussion w/ Sprockets team (https://github.com/sstephenson/sprockets/pull/560) that manifest filename must be explicitly specified by user.
Clarify wording in readme
Update CHANGELOG.md