re-use and freeze string values that never change.
decode 'Content-MD5' checksum as base64-encoded string.

in the RFC the header is defined as:

        Content-MD5   = "Content-MD5" ":" md5-digest
        md5-digest   = <base64 of 128 bit MD5 digest as per RFC 1864>

closes #97
Merge pull request #149 from seancribbs/content-md5

decode 'Content-MD5' checksum as base64-encoded string
Exposes the Webmachine::Application instance to the Adapter

Also randomly assignes a port when running RSpec
Merge pull request #154 from seancribbs/expose_application

Expose Webmachine::Application to adapters
Merge remote-tracking branch 'origin/master' into reel-adapter-fixups