Improve the layout of the Constants help panel

Switches to using a table.widefat instead of a definition list and loses the awkward show example on hover. Also now includes a line showing what the Constant has been set to if it's been set.
Don't allow the plugin to function if the root dir is unreadable.
Zebra stripe the table
Merge pull request #456 from humanmade/unreadable-root

Don't allow the plugin to function if the root dir is unreadable.
Merge branch 'refs/heads/master' into constant-help
Minor improvements to the server info tab
Merge pull request #455 from humanmade/constant-help

Improve the layout of the Constants help panel
Merge branch 'master' into feature/schedule-recurrence-settings

Signed-off-by: Paul de Wouters <paul@hmn.md>

Conflicts:
	classes/class-email.php