docs: Add example of encoding and decoding tokens with RSA (jpadilla#313)

* docs: Add example of encoding and decoding tokens with RSA

Some users have complained that the docs don't make it very clear that
the private key / public key need to be a byte string. This change makes
that clearer by adding an example to usage.rst.

* flake8: Fix a couple of linting errors due to a new version of flake8-import-order

Author: mark-adams

docs/usage.rst

@@ -1,15 +1,30 @@
 Usage Examples
 ==============
 
-Encoding & Decoding Tokens
+Encoding & Decoding Tokens with HS256
 ---------------------------------
 
 .. code-block:: python
 
     >>import jwt
-    >>encoded = jwt.encode({'some': 'payload'}, 'secret', algorithm='HS256')
+    >>key = 'secret'
+    >>encoded = jwt.encode({'some': 'payload'}, key, algorithm='HS256')
     'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzb21lIjoicGF5bG9hZCJ9.4twFt5NiznN84AWoo1d7KO1T_yoc0Z6XOpOVswacPZg'
+    >>decoded = jwt.decode(encoded, key, algorithms='HS256')
+    {'some': 'payload'}
 
+Encoding & Decoding Tokens with RS256 (RSA)
+---------------------------------
+
+.. code-block:: python
+
+    >>import jwt
+    >>private_key = b'-----BEGIN PRIVATE KEY-----\nMIGEAgEAMBAGByqGSM49AgEGBS...'
+    >>public_key = b'-----BEGIN PUBLIC KEY-----\nMHYwEAYHKoZIzj0CAQYFK4EEAC...'
+    >>encoded = jwt.encode({'some': 'payload'}, private_key, algorithm='RS256)
+    'eyJhbGciOiJIU...'
+    >>decoded = jwt.decode(encoded, public_key, algorithms='RS256')
+    {'some': 'payload'}
 
 Specifying Additional Headers
 ---------------------------------