Add VALUE_AS_NUMBER property to ol.dom.Input

This should be used when one want to bind a number instead of a string.
Fix bind-input example to use valueAsNumber
Add a temporary html5 externs file

This is needed because the current version of closure compiler we use does not
include externs for valueAsNumber which we now use in ol.dom.Input.
So this backports patch from issue:
https://code.google.com/p/closure-compiler/issues/detail?id=1060

This patch should be reverted when the next closure compiler release is out
with valueAsNumber externs.
Merge pull request #898 from bbinet/input-valueasnumber

Add support for binding input value as number