adding specs for rels/alternates
modifying existing test JSON output to add blank rel hashes

rel: {} is required by the spec even if it is empty
modified collection.rb to parse rels and alternates

in order to parse relative URLs in rels, we need to parse the base/@href of
the page if there, or to pass the URL in, so added the relevant stuff to support
that.
updated README to move rels into supported section
omitting the nil - breaking the Travis build
Merge pull request #12 from tommorris/rels-alternates

Added support for rel and alternates