Make SerializerBuilder<TObject>.GetTargetMembers() stricter (fix #28)

## Overview
* Fix a bug #28.
* Suppress serialization following members:
  * properties which does not have either public getter or setter.
  * fields which is `initonly`.

## Concerns
* It might be viorates the [collection serializing strategy](https://github.com/msgpack/msgpack-cli/wiki/Serialization-requirements#handling-collection).
* I don't know the status of the unit tests.
If there are problems, please reject it. This modification may be optimistic.