[lib] Fix issue where empty error would cause failure.

Sometimes, the PhoneGap Build API response data will include an "error"
key that is empty. This should not be caught as an error.

Example:

    {
        "error": {}
    }
[lib] Add image support.
[lib] Reposition api.request method.
[lib] Document the API constructor object.
[lib] Add multipart/form-data support.
[lib] Support response codes 201 (created) and 202 (accepted).
[lib] Add file stream support.