Moved ExecutionEnvironment to main project

Move ExecutionEnvironment to main Fixie project to make it available to
other runners.
Merge pull request #49 from JohnStov/Move-ExecutionEnvironment

Moved ExecutionEnvironment from Fixie.Console to Fixie, so that it can be used by other runners.
Add enum ConstructionFrequency.
ClassBehaviorBuilder exposes a ConstructionFrequency value which describes the convention author's choice of construction frequency.
Rename *PerTestClass to *PerClass, for consistency.