Add "importPaths" entry to package.json

Building gl3n separately using "dub generate visuald" currently fails because DMD cannot find the import files. This entry should fix it - see also https://github.com/rejectedsoftware/dub/issues/136
Merge pull request #21 from s-ludwig/patch-1

Add "importPaths" entry to package.json
Merge remote-tracking branch 'upstream/master'