Adds support for contact requests

Title says it all :) One can now add, accept and deny contact quests with:

```ruby
# Sending a contact request
# :message is optional
client.create_contact_request(:user_id => "Your_Id", :message => "Hey, become my newest contact!")

# Accepting a contact request
client.accept_contact_request(:user_id => "Some_User_Who_Sent_You_A_Request")

# Deny a contact request
client.accept_contact_request(:user_id => "Some_User_Who_Sent_You_A_Request")
```