Miscellaneous README changes

- Use Shields.io SVGs for badges (better appearance on retina displays).
- Inline Travis and Coveralls badges with downloads and version badges.
- Rephrase summary sentence (taken from TARDIS' GSoC 2015 description).

README.rst

@@ -2,27 +2,18 @@
 TARDIS
 ******
 
-.. image:: https://pypip.in/v/tardis-sn/badge.png
+.. image:: https://img.shields.io/pypi/v/tardis-sn.svg
   :target: https://pypi.python.org/pypi/tardis-sn
-
-.. image:: https://pypip.in/d/tardis-sn/badge.png
-  :target: https://pypi.python.org/pypi/tardis-sn
-
-
-TARDIS is a code to create synthetic spectra for supernova ejecta
-
-Documentation at http://tardis.readthedocs.org
-
 
-Travis Build Status
--------------------
+.. image:: https://img.shields.io/pypi/dm/tardis-sn.svg
+  :target: https://pypi.python.org/pypi/tardis-sn
 
-.. image:: https://travis-ci.org/tardis-sn/tardis.png?branch=master
+.. image:: https://img.shields.io/travis/tardis-sn/tardis/master.svg
   :target: https://travis-ci.org/tardis-sn/tardis
 
-Test Coverage Status
---------------------
-
-.. image:: https://coveralls.io/repos/tardis-sn/tardis/badge.png
+.. image:: https://img.shields.io/coveralls/tardis-sn/tardis.svg
   :target: https://coveralls.io/r/tardis-sn/tardis
 
+TARDIS is a tool that creates synthetic observations (spectra) for exploding stars (supernovae).
+
+Documentation can be found at http://tardis.readthedocs.org.