Fixing wrong mapping of StaleElementReferenceException to error code 13.

It is supposed to be 10.
Calls to "element.tagName()" should lowercase string.

This is based on WireProtocolSpecs.
There is no section in the W3C WebDriver specs about
this yet.
UUID should only be used for Native Element's ID.

Atoms already produce an ID for WebElements and that should
be the one used.

Fixes #371