Introduce thread safety to SAML schema validation

Per the references below, use of `Dir.chdir` is not thread-safe. This usage was causing exceptions to be raised when running on Puma and in other multi-threaded environments.

This patch also moves the schema read up to a class instance, as this data is static and does not need to be read every time an assertion is validated. This boosts performance, especially in environments with higher throughput.

Thanks to @dannyb for the assistance!

References:

* https://www.ruby-forum.com/topic/165079
* https://bugs.ruby-lang.org/issues/9785
* http://www.justskins.com/forums/working-directory-in-thread-42304.html
* http://www.ruby-doc.org/core-2.1.5/Dir.html#method-c-chdir
@daniel-g what error will this raise? If you check the previous _line 20_, it was doing exactly the same thing. The tests are also passing, so I'm hoping I can trust those. 

My goal is not to change all the functionality, my goal is to solve the problem given the tools available.