Added pngToPdf function to Massage

- Takes in a png and a DPI and converts to a PDF
- 100% Coverage
Change pngToPdf to support all image types
Merge branch 'master' of https://github.com/lob/massage into png-to-pdf
Rewrite method to use callbacks

- 100% coverage all day every day.
update package.json
Add in return statements to test.js
Rearrange order of tests
Merge pull request #12 from lob/png-to-pdf

Added pngToPdf function to Massage