Refactor testing to use unittest fixtures #42 #40 #37

* Fixes #42

  * Removes NereidClient
  * Removes FailFastTextTestRunner
  * Removes TestingProxy and all registered Proxy functions
  * Adds NereidTestCase from which all TestCases can inherit from

* Fixes #40

  * Drop contrib.testing.xmlrunner

* Fixes #37

  * Drop TrytonTemplateLoader
  * Use only FileSystemLoader for production
  * Use function based loading fixture for tests