Got rid of to_link_header and changed isEqual

`to_link_header` not needed as the http transport has its own function
for that.

`isEqual` is inconsistent with changes so far.
Removed get_key, get_tag, get_bucket for RiakLink

Replaced the methods with just direct access to the attributes.
Merge pull request #197 from shuhaowu/link-improvements

Link improvements