Implement support for standard Grunt-0.4 file mappings and options.

See https://github.com/gruntjs/grunt/wiki/Configuring-tasks#files and https://github.com/gruntjs/grunt/wiki/Configuring-tasks#options for details.
Retains backwards compatibility with existing grammar/outputFile/exportVar configuration format.
Remove un-needed extend dependency (now using Grunt's built-in method for providing option defaults).
Tests for the new standard Grunt configuration style.

Doesn't test all possible ways to use the standard style since that's obviously tested upstream by Grunt.
Bring README up to date with standard Grunt file mappings and options format.

Based slightly on the README for grunt-contrib-jshint, etc.
Corrects spelling of `grammer` and 3rd example usage.
Fix JSHint complaints about extra commas (and there was me trying to follow the existing style ;) ).

Should also restore unintentionally changed modes.
Tweak default options.

The PEG.js default options specified here are already the defaults inside PEG.js so no need to pass them, less chance of breaking if they change in future.
Old-style exportVar option now takes precedence over any Grunt-standard exportVar option.
Tiny JSHint nag...
Merge pull request #5 from welwood08/patch-1

Implement support for Grunt-0.4's standard file mappings and options format.