Set the global level via the configuration object passed to
log4js.configure or target `all` in the `levels` object.
Changed to only the ‚levels’ option as discussed in #87
Update log4js.js

Overlooked that there was already a var with '[all]'.
Add unit test to check if the level is set on all categories
Merge branch 'set-global-log-level' of https://github.com/RolfKoenders/log4js-node into RolfKoenders-set-global-log-level
Merge branch 'RolfKoenders-set-global-log-level'