Bind custom dom bindings to the object.

Modules using bindings (e.g. ampersand-view) can pass an extra
value to the bindings() call to bind all custom type functions to.
Rename 'obj' to 'thisArg'
Add test for custom binding with thisArg
Add documentation for `this` binding of custom binding.
Rename thisArg to context
Merge pull request #29 from remko/bind-custom-type

Bind custom dom bindings to the object.