Fix localhost version of Mobile.GetMobileDocumentsPath

[deadlyfingers]

Postcard app is now generating string error with Mobile.GetMobileDocumentsPath in 'mockmobile.js'

TypeError: Arguments to path.join must be strings

[vjrantal]

For cross-platform, you could do something like:

var os = require('os'); // This to the top of the file
return os.tmpdir();

[yaronyg]

This is supposed to be resolved by #63

[deadlyfingers]

@vjrantal - I submitted the update as a PR