Improved get and set accessor support

[dsherret]

As was discovered in #160, there are some problems with the current implementation of accessors. It uses an isReadonly property to tell if an accessor only has a get accessor.

Overall, the current implementation is not very good. It would be better to have a kind property with the following enum:

enum ClassPropertyKind {
    Normal = 0,
    GetAccessor = 1 << 0,
    SetAccessor = 1 << 1,
    GetSetAccessor = GetAccessor | SetAccessor
}

What was also discovered is that currently only get and get/set accessors are supported—not set-only accessors. The code will be changed to support this scenario as well.