Edited the tutorial documentation:

1. Made doctests work in both Python 2 and 3 by adding print functions,
   and u-prefixed strings where necessary.
2. Corrected typo 'suffix with ~' to 'prefix with ~', and added an example
   showing ~ used as a pretty print prefix.
3. Other grammar / wording smoothed over.
Corrected typo 'suffix with ~' to 'prefix with ~', and added an example showing ~ used as a pretty print prefix. Other grammar / wording smoothed over.
Merge branch 'master' of github.com:ryanpdwyer/pint

Conflicts:
	docs/tutorial.rst

The latest commit is correct.
Merge branch 'ryanpdwyer-master'