Added a mock translation handler and translation factories.

Added a mock translation handler and translation factories.

Creating new translation handlers within the classes that use
translated texts breaks the idea of dependency injection.
Providing such classes with a factory that delivers the
translation handler on demand seems to be more flexible to me.

Horde_Translation_Factory_Gettext also distinguishes between
repository checkout and installed PEAR package. In both
situations the locale directories are placed into different
locations.