Add support for coveralls

.travis.yml

@@ -6,3 +6,5 @@ install:
     - pip install .[tests]
     - pip install coveralls
 script: pytest
+after_success:
+  - coveralls

README.rst

@@ -3,7 +3,8 @@
 PySecretHandshake - Secret Handshake in Python
 ==============================================
 
-|build-status|
+
+|build-status| |code-coverage|
 
 This module implements Secret Handshake as specified in Dominc Tarr's paper `"Designing a Secret Handshake: Authenticated
 Key Exchange as a Capability System" <http://dominictarr.github.io/secret-handshake-paper/shs.pdf>`_ (Dominic Tarr, 2015).
@@ -13,3 +14,6 @@ Key Exchange as a Capability System" <http://dominictarr.github.io/secret-handsh
 .. |build-status| image:: https://travis-ci.org/pferreir/PySecretHandshake.svg?branch=master
                    :alt: Travis Build Status
                    :target: https://travis-ci.org/pferreir/PySecretHandshake
+.. |code-coverage| image:: https://coveralls.io/repos/github/pferreir/PySecretHandshake/badge.svg?branch=master
+                   :alt: Code Coverage
+                   :target: https://coveralls.io/github/pferreir/PySecretHandshake?branch=master