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

1 files changed, 2 insertions, 0 deletions

diff --git a/rsa/pkcs1.py b/rsa/pkcs1.py
index 1274fe3..15e4cf6 100644
--- a/rsa/pkcs1.py
+++ b/rsa/pkcs1.py
@@ -321,6 +321,8 @@ def verify(message, signature, pub_key):
     if message_hash != signature_hash:
         raise VerificationError('Verification failed')
 
+    return True
+
 def _hash(message, method_name):
     '''Returns the message digest.