Remove maskBaseDir and destPrefix options

I've replaced the plugin's current technique of taking an array of
sources and options to generate the destination with support for the
standard Grunt source/destination mapping.

The maskBaseDir and destPrefix options required users to learn
options specific to this task that could be accomplished using Grunt's
standardized file mapping utilities instead. Allowing the user to use
Grunt's file configuration means:
    1. The user doesn't have to learn something new.
    2. The user can weild Grunt's much more powerful options.
    3. There is less code in the plugin.

For example, these are equivalent:

Old way, using makeBaseDir and destPrefix:

    css: {
      options: {
        destPrefix: '<%= pkg.version %>/'
        maskBaseDir: '../web/Content/'
      },
      src: ['../web/Content/**/*','!../web/Content/themes/**/*']
    }

New way, using standard Grunt conventions:

    css: {
      files: [{
        expand: true,
        cwd: '../web/Content',
        dest: '<%= pkg.version %>/',
        src: ['**/*', '!themes/**/*']
      }]
    }