add content-disposition option
Merge pull request #297 from freehaha/master

S3 - add content-disposition option
Add const: Units, StatisticDatapoints, ComparisonOperator
Merge pull request #298 from gronpipmaster/feature/cloudwatch-units

Add const: Units, StatisticDatapoints, ComparisonOperator
Add preliminary implementation of PutPartCopy in multi.go
Add test for PutPartCopy and add some more options to the call