Added prototype for Document.reorder().
Merge branch 'master' into reorder
Merge branch 'master' into reorder
Added settings.REORDER
Implemented ordering and added integration tests.

TODO:

- remove comment to properly classify integration tests
- add unit tests for coverage
Heading are now kept when reordering.
Added failing test.
Added missing assert.
Merge from 'master' branch.

Enabled failing integration tests.
Incorporated Level class into reordering.
Test cleanup.
Added '--no-reorder' option.
Cleanup.
Merge pull request #38 from jacebrowning/reorder

Add Document reordering during validation.