More template fields & categorization for email templates.

- Email templates now have more configurable fields
- Rearranged form fields in mail send dialog for invoices and quotes
- The templates are categorized as invoice or quote template, improving usability by displaying only the useful templates for a given category
- Fixes #11 

I have some new language strings. Where do you want them?
```
email_template_tags_instructions
First select a text field, then click on a template tag below to insert the tag into the field.
type
Type
pdf_template
PDF Template
```