Simpler polyfill support

- Have a base class that extends a polyfill class
  - makes classmap generation simpler
  - makes typehinting and mock objects simpler
Revert changes to ClassFileLocator

- No longer necessary to do the halt_compiler hack
Revert changes to CSRF validator

- No longer necessary
Added "build/" to gitignore

- To allow running ant build without prompting for missing files
Updated README

- Changes are simpler, so simpler explanation is in order.
Mark all polyfill bases as abstract

- they are not intended to be instantiated directly; they are intended as bases
  for the typehinted class that uses the polyfills.