Expose FetchMeta.headOnly() in FetchObject

Allows you to retrieve the metadata for an object in Riak without the
value.
Merge branch 'gh153-expose-head' of github.com:basho/riak-java-client into gh153-expose-head
Small change to javadoc
Merge pull request #194 from basho/gh153-expose-head

Expose FetchMeta.headOnly() in FetchObject