Add notes and configuration for wkhtmltopdf on Windows

The wkhtmltopdf-binary gem does not work on Windows, so it has to manually be installed and configured. I added a configuration file and a note to the README.

Instructions for Windows users:
* [Manually install wkhtmltopdf](http://code.google.com/p/wkhtmltopdf/downloads/list?q=Windows%20label=Featured) to a path without spaces in order for pdfkit to work.
* Set the wkhtmltopdf.exe path in config/pdfkit.rb.
* Uncomment the `require './config/pdfkit.rb'` line in toggl-invoice.rb.

See [Installing wkhtmltopdf](https://github.com/pdfkit/pdfkit/wiki/Installing-WKHTMLTOPDF#windows)