refactor(te): Remove Class.TEClient.php from core (refs #4607)

Also:
- Refactored autoloader to provide a classExists() static method to check if a
  class is known by the autoloader without triggering a reindexation of all the
  PHP files.
Merge pull request #1112 from eguaj/3.3-A4607

refactor(te): Remove Class.TEClient.php from core (closes #4607)