Promises on .request and node-style callbacks on .reply is unbalanced

[dantman]

The API is confusing.

Using promises is lovely, but envelope.reply appears to be a node-style callback.

I was actually bitten by this, I expected .reply to simply accept my data and ended up having to dig up where an error Firefox couldn't print was coming from.

I would recommend an alternative to .reply that simply accepts a value, paired with another that accepts an error.

[stephanebachelier]

@dantman It was changed in 5bcda57.
Regarding the changes, maybe another way would be to add an reject method, which means:

- `reply` for a valid response
- `reject` for an invalid response

@ifandelse what do you think about this proposal. I agree with @dantman that introducing node style callback is really confusing.

[cordovapolymer]

@stephanebachelier interesting idea, @ifandelse what do you think?