Enable method calls to native objects via method_missing

Native objects no longer need to be wrapped, and methods can be sent
directly to them. A Native object is any object that does not implement
method_missing. The method missing dispatcher then redirects to a
native_send function in the runtime which handles the dispatch to a
native object. The result is always returned, in the exception of null
and undefined, where nil is returned. This is because methods cannot be
sent to these two objects.
Move special native methods to object (for now).

These should end up in a module for expandability.
Remove some commented code
Fix script_loader to use respond_to? for method checking
Enable method calls to native objects via method_missing

Native objects no longer need to be wrapped, and methods can be sent
directly to them. A Native object is any object that does not implement
method_missing. The method missing dispatcher then redirects to a
native_send function in the runtime which handles the dispatch to a
native object. The result is always returned, in the exception of null
and undefined, where nil is returned. This is because methods cannot be
sent to these two objects.
Move special native methods to object (for now).

These should end up in a module for expandability.
Remove some commented code
Fix script_loader to use respond_to? for method checking
Merge branch 'native' of github.com:opal/opal into native
implement String#tr, String#tr_s methods + specs
Add docs for running native example
Merge branch 'string-tr' of git://github.com/mojavelinux/opal into mojavelinux-string-tr
Merge branch 'mojavelinux-string-tr' into native
implement String#clone, String#dup + specs
implement String#lines behavior when no block is given
Merge pull request #247 from mojavelinux/string-dup

implement String#clone, String#dup + specs
Merge pull request #248 from mojavelinux/string-lines-entries

implement String#lines behavior when no block is given
Merge branch 'master' into native
preserve trailing separator for String#lines
Clean up native specs