Write Send-HipchatCard and Associated Test

[cofonseca]

The function Send-HipchatCard needs to be completed. This function should operate similarly to Send-HipchatMessage, but should post a card-style message in a room rather than a normal message. The card should have the ability to be dropped-down to view additional content.

It also needs a Pester test to go along with it.

[cofonseca]

Card Example: https://developer.atlassian.com/hipchat/guide/sending-messages#SendingMessages-UsingCards