Adds server-side encryption to s3 puts

[lydiaauch]

http://boto.cloudhackers.com/en/latest/ref/s3.html

Optionally adds encrypt_key to set_content_from_string(). This is required for writing to s3 buckets/keys that are protected with encryption at rest.

[lydiaauch]

I'm unsure how to test this in your environment because you have to enable server-side encryption on the bucket/key. However, we have successfully tested it on our production systems. Thanks for your helpful package!

[coveralls]

Coverage increased (+0.07%) to 98.529% when pulling 5feb6ba on lydiaauch:dev_sse into d1a7031 on robgolding:master.

[coveralls]

Coverage increased (+0.07%) to 98.529% when pulling 60a0e54 on lydiaauch:dev_sse into d1a7031 on robgolding:master.

[robgolding]

Thank you @lydiaauch! This looks fantastic 😄

Merging now. I'll bump the version and release shortly.