First sketch of DynamicVariable

This is directly modeled after scala.util.DynamicVariable.
Experimenting with observable composable refs

The current implementation has bugs. Fear!
Observable: all green
DynamicVariable and Observable javadocs
New sample to illustrate Observable and DynamicVariable
Documentation for Observable and DynamicVariable
Merge pull request #139 from jponge/wip/more-utils

More container utilities