Re-enable OAuth password grant flow

[andregasser]

Hello @bocops, according to this page, the OAuth password grant flow seems officially supported. I think we should re-enable it, even if there are risks involved when using it.

Also it makes writing integration tests much easier. as new user accounts can be registered with no browsers involved.

What do you think?

[bocops]

Oh, I haven't seen that one until now. Weird that this is not mentioned on the oauth API methods page, where two other grant_types are explicitlcy mentioned.

We should definitely re-enable this if it is supported and especially if it is useful in testing, but should make it very obvious that this should not be used outside of tests and sample apps. Perhaps more or less revert #56, so that the result will be a deprecated and clearly documented method that does this?

[andregasser]

Agree. I will re-enable this as it was before. Created issue #132 for this.