v3.15 release

- Added caching of scripts. This means using more memory but it also means
  executing scripts 45-65% faster! (mod_perl only)
- Added configurability of caching: PerlSetVar PLPcache On/Off (Default: On)
- Changed PLP.pm to use File::Basename and File::Spec
- Added recursion protection for compile-time includes (fixed max depth: 128)