Add status_callback_url to PhoneNumber.update()

The StatusCallback property of phone numbers is unique in that it 
specifies a URL, but the property name does not end in "Url" like 
VoiceUrl, VoiceFallbackUrl, SmsUrl, and SmsFallbackUrl do. As a 
convenience, the purchase method of PhoneNumbers automatically 
assigns any status_callback_url argument to the StatusCallback 
property. This change adds the same convenience to the update 
method of PhoneNumber.
Merge pull request #89 from mgp/patch-1

Add status_callback_url to PhoneNumber.update()