Remove `serviceWeb()` and `serviceMail()` from public API

[adamfranco]

This is just a reminder of things we might want to do for a 2.0 release that already will have breaking changes in the API.

The getProxiedService() and initializeProxiedService() functions should be sufficient entry points for all proxy requests. Removing serviceWeb() and serviceMail() will reduce the number of ways to accomplish the same result, lowering the number of code-paths to test.

phpCAS::serviceWeb() and phpCAS::serviceMail() could be added to a backward-compatibility layer with a deprecation notice.