Added ability to add sprintf variables to JText string

[writing it up now...]
Merge remote-tracking branch 'remotes/upstream/master' into patch-9
Merge remote-tracking branch 'remotes/upstream/master' into patch-9
Merge remote-tracking branch 'remotes/upstream/master' into patch-9
Added ability to use custom non-language string as first part (can be used for concatenating other strings)
Merge remote-tracking branch 'remotes/upstream/master' into patch-9
Merge remote-tracking branch 'remotes/upstream/master' into patch-9
Merge remote-tracking branch 'remotes/upstream/master' into patch-9
Merge remote-tracking branch 'remotes/upstream/master' into patch-9
Merge remote-tracking branch 'remotes/upstream/master' into patch-9
Merge remote-tracking branch 'remotes/upstream/master' into patch-9
Merge remote-tracking branch 'remotes/upstream/master' into patch-9
Fix wrong access check for com_checkin
Merge pull request #1219 from sanderpotjer/com_checkin

Fix wrong access check for com_checkin
Sync Database to Platform 13.1
Implement Database package tests from the Platform
Fix for failing transactions from joomla/joomla-framework@052e7c3ea2b97a13765ab77a3471ea59c4181716
Enable MySQL(i) and PostgreSQL testing on Travis
Expand database test coverage
Correct unit test