Functionality to handle deprecation

[almarklein]

From v0.6 we want to start calling Flexx Beta. From then on we should be caring more about backward compatibility and deprecation paths.

Beyond 1.0:

• Each deprecated functionality should stay available for at least one release. Printing a warning when used.
• Each deprecated functionality should give a sensible deprecation error for at least one release after the functionality is really removed.
• Deprecation warnings/errors should mention how the user should fix thing, and mention filename and linenr of the offending code.

Maybe the deprecation paths should not be measured in releases but in time (e.g. min half a year for warning/error).