Add option for single file freezing using PyInstaller's `--onefile` option

[jmetz]

As above; I attempted a simple hack of switching the the onedir option in _app.py to onefile and commenting out the part that copies modules to a then non-existent directory, but that then failes because the modules aren't present in the single file runtime.

Having checked throuh flexx/util/freeze.py I can see that this is because of the SourceImporter, so sadly a few-line hack won't work on this any thoughts/plans to implement the single file option (possibly bundling a minimal web runtime also)?

[almarklein]

I don't have plans for implementing that myself at the moment. But a PR that implements it would be welcome :)