Store all necessary data in the Faraday connection cache key

Previously, the cache key consisted only of the resource's URL, even though
the Faraday connection was instantiated with headers and Basic Auth values
that might vary across HyperResource instances.
Ruby 1.8.x fixes
Merge pull request #22 from fancyremarker/faraday-connection-cache

Store all necessary data in the Faraday connection cache key
made post et al work without an implicit get
Merge pull request #25 from gamache/link-http-methods

made HR::Link#post (et al) work without performing an implicit get
adjusted faraday_options test
version bump to 0.2.3
fixed digest/md5 bug; version bump to 0.2.4