ios develop fixes

- added ofXml to ios openframeworks xcode project
- fixed small compiler bug with ofLogWarning
Merge pull request #2288 from julapy/ios-develop-fixes

ios develop fixes
ofXml.cpp: fix crash on empty documents when setting values
linux: fix errors from logging commits
Merge remote-tracking branch 'upstream/develop' into develop

Conflicts:
	libs/openFrameworks/sound/ofOpenALSoundPlayer.cpp
	libs/openFrameworks/sound/ofOpenALSoundPlayer.h
linux: fix all warnings
log level printed before module, increases readability
Merge branch 'develop' of git://github.com/openframeworks/openFrameworks into develop