therie: improved expectation system

* no more 'should' methods added to Object, no more 'should:' macro
* `a should-be(b)` -> `expect(a) to be(b)`
* `{ a } should-raise(x)` -> `expect { a } to raise(x)`
* RSpec-like 'matcher' system
  * failure messages defined on proper structures
  * bonus: matcher methods are also not globalized; defined on ExpectTo class
  * bonus: no reason we can't have operator methods; same semantics
    * e.g. `expect(a) to == b` would just be an alias for `ExpectTo#be`

bump to 0.5