[WIP] Add type declarations for the project

[lapanti]

The commit now includes the type declarations as I see it.

Next steps:

• I would appreciate it if you @fragsalat to check that they correspond to the meaning you had (which ones are strings, numbers etc)
• Some kinds of tests would be needed but I don't have any idea, possibly some kind of test-typescript-file that just uses each class and function as intended? See for example https://github.com/redux-observable/redux-observable/blob/master/test/typings.ts

Related to #19

[coveralls]

Coverage remained the same at 87.698% when pulling 9e08576 on Lapanti:typescript-declaration-file into 3d13813 on zalando:master.

[fragsalat]

Looks good to me.