Allow for the configuration of the authenticating HMAC algorithm

[dekobon]

Currently, the authentication HMAC algorithm is always hard coded to MD5 for all new objects created with the Java SDK. However, the Java SDK will support authenticating objects using other modes. This task is for allowing the selection of the HMAC to use for new objections to be configurable.

Also, it should be noted that using any HMAC other than MD5 will result in a serious performance degradation at scale for a theoretical security improvement.

[indianwhocodes]

RFD0071 citing multi-algorithm support for HMAC.