Add Google Cloud Storage support

[beriberikix]

I read #70 but it would be useful to add support for Google Cloud Storage. What would it take to add another provider? I ask because there is now gcloud-node.

[xpepermint]

+1

[niftylettuce]

@beriberikix @xpepermint a pull request for this would be awesome; I can't write it at the moment since I'm working on http://eskimo.io, however here are a few pointers:

• Modify options object to support a provider name perhaps? (you need some way to detect if we need to use knox or if we need to use gcloud-node to upload assets here https://github.com/niftylettuce/express-cdn/blob/master/lib/main.js#L439-L445
• In all (5) instances of S3.putBuffer, replace or add a switch/case to use the proper provider that the user intends to use (e.g. https://github.com/niftylettuce/express-cdn/blob/master/lib/main.js#L178)
• Add deprecation warning for users that are still using the old format for options

[beriberikix]

Seems manageable. @proppy this would be pretty useful for our samples. WDYT?

[proppy]

@beriberikix +1