GCS: Add custom upload path, Content-Type, ACL, Cache-Control, content-encoding options
Move detect_encoding? and encoding_for to provider.rb

And revert removing gcs example at README.md
Merge pull request #145 from keima/feature/gcs-contenttype

GCS: Add custom upload path, Content-Type, ACL, Cache-Control, etc!

Conflicts:
	README.md
	spec/provider/gcs_spec.rb
Merge pull request #153 from travis-ci/test-pr145

Resolve merge conflict with #145