Type declaration to not require optional parameters

[jtfell]

The current type declaration requires that all properties are supplied when creating a new Provider or Request object, when the code supplies default values in the constructor.

See:

• https://github.com/zalando-stups/oauth2-client-js/blob/master/src/request.js#L17
• https://github.com/zalando-stups/oauth2-client-js/blob/master/src/request.js#L30
• https://github.com/zalando-stups/oauth2-client-js/blob/master/src/provider.js#L13

This PR flags the non-required parameters as optional so that using the default values passes type-checking. Without this change, the main example in the README has TS errors.

[Sunjammer]

Bump this. It makes the lib very awkward to use in a strict TS environment.