AWSCredentialsProvider for s3-extentions

- fileSessionCredentials property is added
- AWSSessionCredentialsAdapter is adapter class between aws sdk AWSCredentialsProvider to jets3t Credentials
- FileSessionCredentialsProvider will refresh its session credentials from the file
- S3StorageDruidModule should return different AWSCredentialsProvider on the config settings
A few fixes

- EasyMock instead of Mockito
- FileSessionCredentialsProvider fields should be volatile
- getRestS3Service should create AWSCredentials not a AWSSessionCredentials with fixed credentials
License header and utf-8
update refresh manually every hour
refresh thread should be daemon
Properties.load instead of manual population
Merge pull request #629 from metacret/ISSUE-628

AWSCredentialsProvider for s3-extentions