fix remote authentication when username/password have special characters

CBLChangeTracker and CBLRemoteRequest read credentials out of a URL
object, which will return them in url-encoded form. They have to be
url-decoded before use, or authentication with the remote doesn't work.

(fixes couchbase/couchbase-lite-android#26)
Merge pull request #14 from mjq/fix_escaped_pw

fix remote authentication when username/password have special characters