version 0.2:

 - remove `str` function - just use `repr()` or `String()`
 - make global installation (of global.repr and Object.prototype.toString()) opt-in
 - make require('repr') return a function, rather than a module with `repr` and
   `install` functions.
 - [browser]: just set window.repr, rather than window.Repr and window.repr
 - misc fixes, including cycle detection
build stuff