Remove duplicate posts and tags on import

Issue #806

- Modify sanitize to check for duplicate posts and tags
- Update posts_tags when referencing removed duplicate tags
- Return both data and problems from importData
- Add tests for duplicate posts and tags with fixtures
Created image helper with absolute url option

closes #4231

- Adds {{image}} helper
- Adds image_spec test unit
- Updated {{ghost_head}} to use image helper
Update Sass Grunt task output style

No issue

A recent update to NodeSass changed the keyword for output style from `style` to `outputStyle`.
This updates that, thus making the minfied CSS, actually minfied.
Merge pull request #4380 from PaulAdamDavis/sass-compress

Update Sass Grunt task output style
Merge pull request #4155 from jgable/importDuplicates

Remove duplicate posts and tags on import
Merge pull request #4317 from cobbspur/image

Created image helper with absolute url option
Add pg as an optional dependency.

Closes #1333
- Change PostgreSQL build on Travis from an allowed failure to
  failing the tests just like sqlite3 and MySQL.