Move default encoding to BeyondConfiguration.

Encoding would be used quite much in the future, so it'd be
better to have one shared configuration here.
Implement ScriptableBuffer.

ScriptableBuffer is an implementation of `Buffer` class in Rhino.
It basically provides methods to manipulate buffers with String and
Byte. An example code representing the usage of the class is also
added in the `examples` directory in plugins.