Assertion Standardization

Opening this up for input although there are still a few blanks. As discussed in #7 I've had a go at redrafting the assertion spec with standards language/ guidance in mind:

* https://www.ietf.org/rfc/rfc2119.txt
* http://tools.ietf.org/html/rfc2223
* http://tools.ietf.org/html/rfc2360

Also used these W3C examples:

* http://www.w3.org/TR/json-ld
* http://www.w3.org/TR/SVG/struct.html

__Assertion Spec Notes__:
* blanks include "Status of this Document", "History" and "References"
 * I've left notes in code comments in some parts of the document linking to relevant IETF guidance
 * haven't gone into much detail in terms of context - the History section could maybe go into this, i.e. outlining the decision history, explaining why aspects of the spec are the way they are etc
* have removed the "Setting Content-Type" examples section, thought that might belong in a separate guide
* am assuming the spec may need significant additions pending discussions regarding extensions/ inline objects etc

__General Notes:__
* have removed link to Backpack "todo" page from repo readme
* have redrafted the baking doc but not with standards language etc in mind so it's still in the original format