introduce lint

we want to introduce lint into our workflow.
in order to be able to check lint warnings in our IDEs (without getting 500 errors/warnings), lint.xml files were pushed into our Android projects - some warnings are ignored (unfortunately I cannot comment them in the lint.xml files, because if somebody else wants to ignore another warning and e.g. adds the warning with eclipse right-click->ignore, eclipse deletes all comments and changes the format).
what is ignored:
* contentDescription - similar to CSS, a contentDescription should be added for textual representation of images; in my opinion currently not needed
* IconDensities - therefore we need Icons from the UX team; Issue is created - #670; warning will be enabled if icons are in the repo
* InvalidPackage - this one has shown several warnings with XStream concerning the compatibility with Android - but our XStream version is build for Android and warning can be ignored
* MissingTranslation/ExtraTranslation/Unused resources - since we only provide translations in english and german, other languages can be ignored.
* MissingApplicationIcon and AllowBackup - ignored in test packages
* IconLocation - ignored in UiTest package; no need to provide icons with all densities

other warnings that are not ignored in lint.xml files but within xml files (needed to ignore just one single statement) are commented!!

current Lint checker run: https://jenkins.catrob.at/view/Util/job/Catroid-Code-Checker/69/

since we have 5 warnings, I will set a threshold for PullrequestTest, Catroid-multi-job and Catroid-multi-job-master - 5 warnings is ok, and nobody can introduce new warnings.
If some of our existing warning-issues are resolved, threshold has to be adapted.

One last comment: when accepting pull requests we have to be careful if somebody changes lint.xml files; at least there should be a good reason why lint warnings can be ignored :)