adds preliminary typings for Typescript

[aravantv]

Initial contribution to Issue #26.

Provides:

• (Xstream-only) typing for Collection and pluck
• inline documentation for both functions

Still to do:

• typing for non-Xstream libraries (can it simply be done by typing makeCollection? if not, can typing be factorized as easy as the code?)
• typing for gather and merge

[Widdershin]

Sorry for the delay in response.

This looks really good! I like the union type for adding sources together. I haven't really used generics in TS yet so this is informative.

I can merge this now if you like or I can wait until we've amassed a full definition.

[aravantv]

I would merge it now, cause it's doing the job for me for now, so I can hardly promise that I will have time to improve it much in a close future... Sorry :-(

[Widdershin]

@aravantv great, I'll merge it now. Thanks for contributing this! It's a great start on a definition. I especially like the documentation.