better keychain key names.

The constants were some ending with name, others not. This was
inconsitent. Also confusing because `CKIKeychainCurrentUserName`
made the key look like it was only storing a user name, when in
actuality 'name' referred to the fact that the constant was the
name/key value in the keychain where the current user was stored.
rename domain to baseURL

this makes more sense and is more consistent
because it matches the name of the property on
the CKIClient object.
Merge pull request #39 from jarsen/master

Minor renames and fix ups