Change to mPDF library for HTML printing support
Don't process alpha channel of PNG images
Use consistent styles for printed ticket thread
Merge branch 'develop' into feature/html-pdf
Use <pre> elements for text emails

This provides better compatibility with mPDF, which doesn't support the CSS
white-space:pre-wrap property
Complete lint testing for mPDF
Merge pull request #44 from greezybacon/feature/html-pdf

Support HTML printing  to PDF

Reviewed-By: Peter Rotich <peter@osticket.com>