Allow symlinked standards

To symlink a standard might be useful while developing standards outside the codesniffer repository. This is not possible however, as sniffs not explicitly mentioned in the ruleset will be ignored.

This patch enables symlinked standards for PHP 5.2.11+ and 5.3.1+.

Use cases I could think of:
* multiple phpcs-versions to test against
* clean code path separation of phpcs and standard
* may install standard through composer from within the standards repository

Can not see a real benefit for production use, but no disadvantage either.