Parser refactoring

This PR is a complete refactoring of how we parse .ini files. The `StringScanner` class is no longer used. Instead we parse on a line by line basis. This enables us to read files one line at a time instead of loading the entire contents into a string and then parsing.

In order for this new parser to work, I've used [negative lookbehind](http://www.regular-expressions.info/lookaround.html#lookbehind) regular expressions. These are only available with Ruby 1.9 and above. This release of inifile will no longer work with Ruby 1.8.

I've also attempted to address a few of the open issues: 
closes #15
closes #16
closes #18
closes #26 

More changes to follow.