User adds phone number to pool

[mattreduce]

In addition to any future integrations where Sockdrawer can help speed up the provisioning of new virtual numbers for SMS and calls, allow users to maintain a pool of numbers both virtual and not that can be assigned to a given Sock.

We can expand on this feature to warn users when they try to re-assign a given number to a different Sock, suggesting that they create a new one instead, unless they're really sure.

Attributes:

• Number
• Carrier
• Rate/cost?
• Expiration date (we can build in reminders for trial SIM cards that are only good for a week, for example)
• Details/usage
• Kind: VOIP or "real"