Update and synchronize ehtml documentation
yaws_api: Fix ehtml {check, _, number()} attributes

Attribute {check, Key, 10} should expand the same way as {Key, 10}.
yaws_api: Allow binary attribute values in ehtml

Binary attribute values are already supported in exhtml.
yaws_api: Cosmetics (reduce redundancy)
Merge pull request #183 from weisslj/ehtml-attribute-fixes

EHTML: Fix small bug, allow binary attribute values and update documentation