Update db to include accessibility_errors

* change `valid` to `html_errors`
* add `accessibility_errors`
update keys
add a script to install the db
add an accessibility logger

Note -- still testing
Add accessibility to json output
default to 'unknown' if error
remove `code` and `site_id`

They are unused
Merge pull request #14 from mfairchild365/master

add an accessibility logger