enable examples from local files again

[maxheld83]

the fix to #298 seems to break all examples based on local files, probably because sth with the standardizePath() screwed things up.

Help:

• http://stackoverflow.com/questions/13463103/inst-and-extdata-folders-in-r-packaging
• http://stackoverflow.com/questions/24867153/vignette-cant-find-data-files-during-devtoolscheck