support declaration merging

[nadako]

Subj. Somehow. Gotta read the spec about how does it work. 😉

[frabbit]

that will be fun ;)

[Simn]

It shouldn't be that hard once someone figures out how it actually works.

[nadako]

https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#72-declaration-merging

looks like that we should simply merge fields in a structure, there's no specific rules about usage of types between duplicate declarations and it's always resolved to a merged type: http://bit.ly/1B4yvE8