adding boolean as a data type

[teonimesic]

adding a 'boolean' data type that can be edited with a check box.

I made boolean value to be either 'true' or 'false', since the raw field is typecast as a String.

I've tried to remove the String typecast before from the raw field, but it resulted in so many conflicts with Integer and YAML types that I decided to just use 'true' and 'false' for boolean values and leave the typecast on raw field as is.

[glebtv]

I agree that a proper boolean field is good to have, but we usually just use enabled field for boolean (and not use the value). Is there any reason you can't you use it?