Transient cache verify_database_present for 24 hrs

cache both $database_message and  $uninstall_message and reuse if possible, assuming database availability unlikely to change
Use static method is_network_activated()
Updating repo links
Readme tweaks for Travis URL
Another readme tweak
Adding new assets and tested up to for WP 4.0
Updating readme
Merge master into develop
Updating plugin search tags
Merge pull request #605 from adamsilverstein/cache-verify_database_present

Transient cache verify_database_present for 24 hrs
Merge develop into 2.0.0-beta