20 accept content attr globally

[chaals]

fix #20
fix #38
fix #39

Changes the algorithm for finding values, to add @content, and make data/meter/time default to their text content if there is no attribute to use.

Re-work the content models and examples to take account of this.

And note it in the changes section

[msporny]

As @Hixie mentioned in #20 - "@content fared poorly in usability studies.", so my +1 to add it is now a +0. Do we either have 1) Data from @Hixie noting the usability studies (doubtful, as Google has not released this data to the public AFAIK), or 2) Demonstration that "@content" is being used across X million sites in Microdata markup.

If we don't have data for either, we're just flying by the seat of our pants (which is usually a bad thing to do spec-wise).

There is another option here, which is to fix the schema.org examples.

[gkellogg]

Does Microdata need to say something about the HTML content model being updated to allow the @content attribute to be used universally?

Tests should probably look for potential interference between @content and @src, @href, @data, @value, and @datetime.