process individual files instead of concatenating

Addresses two issues:
https://github.com/dwightjack/grunt-premailer/issues/16
https://github.com/dwightjack/grunt-premailer/issues/17
Merge branch 'amclin-master' into hotfix/tmp-src
fix tmp path and file mapping
Merge branch 'hotfix/tmp-src'
Merge branch 'hotfix/tmp-src' into develop
Merging #18 to fix #16 and #17.
Merge branch 'release/0.2.8'