Add default value

Change documentation
Add default value for direct mapping
Implement +storageBehaviorForPropertyWithKey:
Move property key reflection to +initialize
Negate spec description
Defer reflection
Extract the MTLModel protocol
Add spec for conforming models
Document property storage behavior
Improve documentation of MTLModel
Rename generateAndCachePropertyKeys method
Improve documentation of MTLModel
WS
Pass in attributes to determine storage behavior
Look up associated objects less frequently
Remove attribute argument
Fix TomDoc style
Only consider weak properties transitory
Merge branch '2.0-development' into property-behavior
#238: Log errors on encoding/decoding with the key name.
Merge pull request #241 from dblock/log-error-on-encoding-decoding

Log errors on encoding/decoding with the key name.