rsa.pkcs1.verify() should return True when successful

- when verification passes verify() will return True, instead of
  None. If verification fails the function will still raise a
  rsa.pkcs1.VerificationError for legacy purposes.
- update the docs to note that the verify() function returns True
  when successful
- write unit tests to verify this new behavior

This commit passes all build tests:

    Ran 44 tests in 1.217s

    OK

2 files changed, 2 insertions, 2 deletions

diff --git a/tests/test_bigfile.py b/tests/test_bigfile.py
index 974da8b..86bcbba 100644
--- a/tests/test_bigfile.py
+++ b/tests/test_bigfile.py
@@ -51,7 +51,7 @@ class BigfileTest(unittest2.TestCase):
 
         # Check the signature
         msgfile.seek(0)
-        pkcs1.verify(msgfile, signature, pub_key)
+        self.assertTrue(pkcs1.verify(msgfile, signature, pub_key))
 
         # Alter the message, re-check
         msgfile = BytesIO(b('123456sybren'))
diff --git a/tests/test_pkcs1.py b/tests/test_pkcs1.py
index 55098e2..d5882df 100644
--- a/tests/test_pkcs1.py
+++ b/tests/test_pkcs1.py
@@ -64,7 +64,7 @@ class SignatureTest(unittest2.TestCase):
         signature = pkcs1.sign(message, self.priv, 'SHA-256')
         print("\tSignature: %r" % signature)
 
-        pkcs1.verify(message, signature, self.pub)
+        self.assertTrue(pkcs1.verify(message, signature, self.pub))
 
     def test_alter_message(self):
         '''Altering the message should let the verification fail.'''