Encapsulate a value (object and keypath) into a BNDValue object.

working on #4

Hooray more boilerplate! Will go down when factories are added.
Changed BNDTrigger from a class to a protocol.

working on #4
Renamed BNDValue to BNDProperty.

working on #4
Fix spec name.

working on #4
BNDProperty is also defined as a protocol, so that other objects may appear to be properties.

working on #4
Removed property definitions from BNDProperty as they are unnecessary.

working on #4
Implemented a transformer for converting strings to numbers.

closes #4
Updated README for 0.2.0.