IVar: basic implementation.
IVar: replace Contract.
IVar: initial value.
Better YARD documentation.
IVar: Future extends IVar.
IVar: dataflow needs IVar, not Future.
Merge branch 'master' into ivars

Conflicts:
	lib/concurrent/future.rb
Merge pull request #27 from jdantonio/ivars

Implementation of IVar
Merge branch 'master' into better-docs