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.