Remove list of PHP constants, use regex instead

The rationale here is that maintaining a complete list of constants is
impossible, considering the many extensions out there. We're now simply
marking all capped symbols as constants.

Also matching the class name of a static method call as constant,
c++-mode does the same.

Some remaining todo's:
 * make the namespace separator behave / look like :: in c++ mode
 * fix issue-53 or remove whitespace stripping from php-mode (recommend
 ws-trim-mode)