Add new cop VariableInterpolation

It tracks uses of old school variable interpolation like
"test#$global_var".
Fix #59 - proper handling of interpolated vars
Fix broken build
Add new cop Semicolon

It discourages the use of semicolons to separate expressions in the code.
Really fix broken specs
Add a new cop FavorSprintf

It's a bit of a work in progress. Currently it does not work with
strings that have embedded strings in them. The lineno logic should be improved.
Add a bit of explanation to FavorSprintf
Code style - use sprintf instead of %
Add a pending example for the sprintf cop

Currently the cop does not work for strings with embedded expressions
in the them. This example should serve as a reminder of that.
Add a contributing section to the README
Fix a typo
Add Changelog
Add new cop Alias
Fix #42 - nasty interpolation bug

If an error message contains `%` sprintf gets confused, so we need to
take special care to escape such messages properly.
Add a spec for #42
Update Changelog
Fix issue link
Update Changelog for 0.5.0
Version bump to 0.5.0
Regenerate gemspec for version 0.5.0