Added some options and upgraded version of aws-java-sdk

The current version of the S3blobs module generates a warning during file upload that since the content-length header is not set, it will buffer the entire file in-memory before uploading.
We sometime need to upload biggish files (8MB+) - this didn't bode well for memory consumption. So I added a content-length field that can be passed prior to caliing the "set" method - seems to work well.

I also added an optional configuration property  called "s3.store.enabled" which checked in S3Blobs during app startup - handy if you're in a dev/test environment and don't want calls being made to S3 during app startup.

Finally I upgraded the version of the SDK to 1.3.6, which seems to offer some improvements (I noticed other people's forks also contained this)

P.S. - I'm pretty new to merge requests, wrting Play modules etc. - so any comments/feedback about improvements would be much appreciated!