Feature - Send Private Message

support sending private messages to a user

### Expected Behavior
send-hipchat -message "foo" -user "email@example.com" -apitoken "xxx"

### Current Behavior
no feature exists

### Possible Solution
PR #9 

### Steps to Reproduce (for bugs)
N/a

### Context
Don't want to spam a room with personal updates

### Your Environment
N/A