Bump requests version up to 2.0.1

Change-Id: I60d31b86a8df2e5cb785f367140de4567ff32e10
Bump to requests 2.0.1 also in setup.py

Change-Id: I41828e0c181b53604ef84f423267f1c45ca6d7b4
Fix NameError in REST API example

Change-Id: Ia72639e9579e6a438b520b50a799f18184de206b
Add method to get username and Gerrit version

If the username is not given in the constructor, it is detected
from the ssh config.

Add a method that allows the client to get the detected username
that was used to connect to Gerrit.

Change-Id: I15747d1446b07ce6b250f294fe72842a1be50a6c
Add method to run a gerrit command from the client

Change-Id: If1863c06619a11f93efc7c25c7c33c749950d6a5
Add username field in the account data

Change-Id: I91b90d26fde775d5d58ee127fd9015757090a978
Simplify REST API error handling

Remove GerritRestAPIError.  Raise requests.HTTPError when a request
results in an HTTP Error.  Raise ValueError in other places.

Change-Id: I25188f64161a6ca8159e7777e3f288f775463c25
Handle errors in REST API example

Change-Id: I0fc676f74f6e41c25d7f9f8a504e6a507a5bb43a