Fixes login-issues via JSON-API

When using the JSON-API the translations-Object is not set in a global
scope but inside a function and is therefore not globally available.
Merge pull request #153 from heiglandreas/fixLoginViaJsonAPI

Fixes login-issues via JSON-API
Fixed PHP warning for empty list in export extension. (refs #160)
Merge branch 'master' of github.com:kimai/kimai
Finally dropped support for PDO. Updater.php migrates autoconf.php to MySQL.
Fixed reference to wrong translation entry in table PDF export.
Fully renamed task to activity, as it already was internally.
Indicate for non-active tabs if there is an error within that tab. (closes #162)