Disable validation when *assert* is false

Initial work to disable validation and elide schema checkers from emitted schema
fn/defn code. When *assert* global var is false, treat all schema fn's as
:never-validate as long as they are not tagged :always-validate.

Adds a ClojureScript build to set *assert* false at compile time and tests
alongside normal CLJS tests.

Updates tests to not assert thrown exceptions from validation errors when
validation was disabled.

Next step is to consider whether we want to create our own flag rather than
coupling to *assert*.
Add schema-specific flag for eliding validation
Change *compile-fn-validation* to atom
Elide validation check when not compiled
Merge pull request #159 from Prismatic/elide-validation

Add support to elide fn validation at compile time