issue #1: make Type::Boolean cast via truthy/falsy
Merge branch 'feature/boolean-casting' into develop
add list of built-in type definitions to readme
update readme to reflect UintXX as elements of the set of natural numbers (ℕ)
Add TOC to README usage section
break spec helpers out into their own file
break scalar and collection builtins out for clarity
remove unused remnanant of abandoned WIP.
short-circuit casting if input is valid
allow type definitions to raise sensible exceptions in validate
Add travis-ci config and make rbx specs pass
drop 1.8.7 and 1.9.2 support, fix travis config
fix build for mri >= 2.0
actually drop 1.8 support [ci skip]
Fix bug in JRuby with Type::Int64.

In jruby, `-(1<<63).pred` overflows to `(1<<63)`,
but `-(1<<63) - 1` is correct.
Update Changelog [ci skip]
Fix typo in contribution guidelines [ci skip]
fix typo in changelog
version bump to 0.2.0
Merge branch 'release/0.2.0'