added get_pdf method

[Greyoxide]

Hi this is my first ever pull request so let me know if I've done something wrong.

I've implemented a get_pdfmethod as described in issue number 62. I've also added a test to the test suite which tests this new method against the invoices endpoint. I've also tested the new method against the development mode on my production app by downloading a pdf copy of an invoice, estimate, and purchase order.

In addition to the get_pdf method appended to qbo_api/api_methods.rb, I've also adjusted the response method found in connection.rb. Given that application/json is the default content type, I've added a check to determine if this case is an exception to the default content type.