Revert "Prevent users from using any system-property names in model definitions"

This reverts commit 7ecdf0932ebe5a2bac81127e5d46dfefbe50432c.
Revert "Error when trying to define a "type" property"

This reverts commit 852df3c784c22fb354a432a725be4034cc0ad2fd.
Merge branch 'v0.5'