Add a callback when writing to the filesystem.

The Workbook write function can now be called in 3 ways:

`workbook.write(filename)`
    will write the workbook to a file asynchronously.

`workbook.write(filename, response)`
    will write the workbook to a node response object.

`workbook.write(filename, cb)`
    will write the workbook to a file, and then call the supplied callback
    function when writing is complete.

This fixes #4.