Add destpath argument to 'process' callback

This is a change to the "process" option to pass the destination path to the callback as well as the source path. Where before it was `process(content, srcpath)`, now it is `process(content, srcpath, destpath)`.

I'm a little confused why the destination path was never included as an argument in the first place, both for this plugin and for the main grunt.file.copy API - both the source path and destination path should be fixed and available at the time the callback is invoked, and they could both be useful. If it's assumed that the destination path will be in scope to the callback's definition (which might be reasonable for grunt.file.copy, not as much for this plugin), why would the source path be included?

Note: One change extraneous to the "process" option is a fix for the tests on OS X, excluding dotfiles that are generated by Finder. (Lines 10-28, test/copy_test.js)