PreMailer.Net 2

Opening this pull request early, to track features and progress for PreMailer.Net 2.

Complete re-write to support external styles, honor CSS specificity, and add more pre-mailing features as supported by the "original" ruby gem.

## New features
- [ ] Honor CSS specificity #2 
- [ ] Support `<link>` elements #8 

### Additional pre-mailing features #5 
- [ ] Relative paths are converted to absolute paths Checks links in `href`, `src` and CSS `url('')`
- [ ] A plain text version is created

## Bugs
- [ ] Support background URLs #12 
- [ ] Support composed fonts #9