Lock Capability - closes #11

[benfrancis]

Lock capability with LockedProperty and TargetLockedProperty.

Notes:

• I made TargetLockedProperty a string enum rather than a boolean to be consistent with LockedProperty but also because I can imagine feasibly adding other states later (e.g. "doublelocked")
• I haven't added a property to store key codes. Partly because we don't yet support object properties, but also partly because I think it might require some more thought, e.g. around whether you actually want to send key codes in plain text over the Internet, and what's the best way to set and store them. We can extend the schema with this later, or potentially create a separate key pad schema of some kind.

Question:

• Should TargetLockedProperty be required or not? Might there be a need for a read-only lock which you can read the state of but not actuate? We'd need to figure out how you would represent that in a UI.

[mrstegeman]

LGTM, other than the one typo. Nice and simple.

[mrstegeman]

typo: "uknown"