Clean up test runner

Test code doesn't need to manually construct a TestSuite and a
TextTestRunner, the unittest module has a discovery function that does
all this for you.

Delete all of the manual logic from tests.py, replace it with the two
lines to bring in the doctest unit tests, and update the makefile to
run the unittest discovery.

makefile

@@ -10,7 +10,7 @@ help:
 	@echo
 
 test:
-	python tests.py
+	python -munittest
 
 coverage:
 	coverage run --source=jsonpointer tests.py

tests.py

@@ -7,6 +7,7 @@
 import unittest
 import sys
 import copy
+import jsonpointer
 from jsonpointer import resolve_pointer, EndOfList, JsonPointerException, \
          JsonPointer, set_pointer
 
@@ -410,23 +411,6 @@ def test_mock_dict_raises_key_error(self):
         self.assertRaises(JsonPointerException, resolve_pointer, doc, '/root/1/2/3/4')
 
 
-
-suite = unittest.TestSuite()
-suite.addTest(unittest.makeSuite(SpecificationTests))
-suite.addTest(unittest.makeSuite(ComparisonTests))
-suite.addTest(unittest.makeSuite(WrongInputTests))
-suite.addTest(unittest.makeSuite(ToLastTests))
-suite.addTest(unittest.makeSuite(SetTests))
-suite.addTest(unittest.makeSuite(AltTypesTests))
-
-modules = ['jsonpointer']
-
-for module in modules:
-    m = __import__(module, fromlist=[module])
-    suite.addTest(doctest.DocTestSuite(m))
-
-runner = unittest.TextTestRunner(verbosity=1)
-result = runner.run(suite)
-
-if not result.wasSuccessful():
-    sys.exit(1)
+def load_tests(loader, tests, ignore):
+    tests.addTests(doctest.DocTestSuite(jsonpointer))
+    return tests