Render return types + minor changes

[perludum]

• Found and fixed the bug for the missing return types in the parser, it turned out to be easy :)
• Shifted the dependency on ps.constants.d.ts from index.d.ts to ps.types.d.ts (via app.ts). Otherwise Intellisense gives a hundred errors in ps.types.d.ts, because it can't "see" the kinds of return types.
• The abstract type "Layer" is not documented in the Adobe reference and was missing in one place. A "Layer" can be of type ArtLayer or LayerSet. Added this to index.d.ts.
• Had to do 2 minor changes in the assets, because it rendered "Makes" and "Contracts" as return types, though I cannot find no reason why it does so.

[felixSchl]

This is awesome. Thank you for preparing this. I cannot see anything wrong with it and am going to merge it right away. I'll also set you the commit bit after this so you are free to commit more changes if you feel you need to. Are you relying on npm at all when installing these typings? We can publish your changes under the current package name for 0.4.0, but what do you think about #6 ?

[perludum]

Great, thank you! I'd like to rely on npm when installing these typings, though currently there are still some (partly manual) fixes that need to be done. Some of them are related to https://github.com/felixSchl/photoshop.d.ts/issues/6 (Extendscript typings). I'll look into it and hopefully we can publish it to the @types package in a later version.