Fix 'Attachment model missing type definition for' errors

[jgedarovich]

While testing out this library I found that I received 'Attachment model missing type definition for' errors until making the changes to the Attachment Resource to include the following additions:

        mrkdwnIn:
            type: array
        titleLink:
            type: string
        authorName:
            type: string
        authorLink:
            type: string
        authorIcon:
            type: string

Fixes #66

[jderusse]

:+1:

[RoccoHoward]

I've imported this package into my own package and incorporated your changes.

https://github.com/bluora/php-slack-api