Use default Exception class when generating PHP classes.

This was forgotten when implementing PHP 5.5 support for release 2.0.0.
Fix generation of Enum constants which resolve to PHP primitives.

We only need to check for keyword usage in enum names. Type validation/conversion is not necessary here.

Added functional test to verify fix and ignore generated test code.

This fixes #12.
Merge pull request #67 from kasperg/fix-enum-constants

Fix generation of Enum constants which resolve to PHP primitives