Hardcode adapter dependencies in gemspec

We will be able to use platform conditions when Bundler will support
platform-aware Gemfile.lock.
Merge branch 'master' into adapter_dependencies
Remove dependency_manager as it is useless now
Merge branch 'v1.0' into adapter_dependencies

Conflicts:
	lib/listen/adapter.rb
	lib/listen/adapters/bsd.rb
	lib/listen/adapters/darwin.rb
	lib/listen/adapters/linux.rb
	lib/listen/adapters/polling.rb
	lib/listen/adapters/windows.rb
Remove wdm gem from gemspec

As the wdm aren't installable on other vm than MRI >= 1.9.2, it should
be added mannually on the Gemfile. Too bad!
Merge pull request #100 from guard/adapter_dependencies

Move adapter dependencies to gemspec (2)