Add support for a PURPLE_LOCALE_DIR environment variable so we can set the

locale directory in a dev environment.
Added purple_get_locale_dir() which will check the environment variable or

fallback to PURPLE_LOCALE_DIR.
Removed the old PACKAGE and PACKAGE_NAME constants and replaced them with

GETTEXT_PACKAGE which is much more self explanatory.

Compiled with finch enabled and verified no issues.