Add support for Webhooks. Extracted processing an asset file from create method into separate method.
Merge pull request #16 from contentful/add_webhooks

Add support for Webhooks. Extracted processing an asset file from create method into separate method.
Merge branch 'master' into image_url

Conflicts:
	spec/lib/contentful/management/asset_spec.rb
Added specs for image_url method from  Asset class