Runtime: Extract cap normalization logic into a separate method - parseCaps().

Accepts comma delimited string and converts it to an object.
Runtime: Run can() method returned from getRuntime() in runtime's object.
XHR/FileInput/Image: Normalize required_caps if represented by comma delimited string.
Runtime: Make parseCaps() static.
Image, HTML5: Survive the case when file cannot be read for some reason or in wrong format.
FileInput, HTML5: For IE only, reset file input's value by reinserting input element into the DOM. Related #811.

In IE input[type="file"] is read-only so the only way to reset it is to
reinsert it.
Image: Check if options are passed before parsing required_caps on them.