observe: Use typescript index types

[Strate]

This enables better typecheck and inference.

[coveralls]

Coverage remained the same at 96.226% when pulling 389edb8 on observe_ts_index_types into bc9efb1 on master.

[hccampos]

This will break compatibility for everyone using TS < 2.1 if I am not mistaken. Would probably warrant a major version bump

[mweststrate]

Great @Strate! Thanks for pointing out @hccampos, will indeed be breaking for TS 2.0 users. Will merge it as soon as MobX 4 is nigh. Note that MobX 3 won't be as long lived as MobX 2, so shouldn't take too long :)

[mweststrate]

Merged into #1321, closing here.