Rewrite of 4.1.11 Attachments

Well-structured original to begin with, but this section also contained
many sentences that were either descriptions, procedures or a
combination of both.(For example, the paragraph 'Transmission Format' in
the original). I took a stricter approach in separating that.

The Rationale-paragragh is also aimed at non-techies and as such
contained much too complex content. This was placed elsewhere in the
section.

Took out he unclear term "common requirements", this does not define
well what the requirements relate to (statements, attachments, LRS,
client?) and put those under the header "requirements for statement
streams that include attachments".

Made a literal copy-n-paste of the code examples, but they didn't look
good when previewing. Sorry if they turn out to be wrong on Github.
Someone more experienced with Github should please correct this.
Merge branch 'master' of https://github.com/adlnet/xAPI-Spec
Activity and Verb Metadata

* changed metadata to JSON from XML within spec
* defined JSON format
* allowed any type of metadata at URI (LRMI, microdata, RDF, etc) (for activities, not listed, but implied)
* did the same for verb, etc... metadata -- except talking about "any format", do we want that here?
* ADD ‘url’ field to activity definition for ‘read more’ or ‘launch

Still todo:
TODO: 5 - create 2 content types (application/json+XXX)
TODO: 6 a- it may simply be a human readable document
Some changes based on feedback

* weakened: "the LRS SHOULD replace its internal representation of that activity definition with the loaded definition" to take into account statements possibly having more information/translations
* removed The LRS MAY change its internal representation... since it's redundant
* in general identifier metadata section, clarified activity definitions have their own format
* clarified that metadata SHOULD be available only for URLs coined for the purpose of use in this specification (in order to avoid the implication that URLs without metadata should be avoided)
* In general identifier metadata section, clarified that metadata may come from other sources or formats as well, particularly translations
remove voided from statement properties list
Merge pull request #14 from paul-esch/master

Fork Paul's repo
Merge pull request #15 from paul-esch/master

Fork Paul's repo
remove reference to voided flag in immutability discussion
add reference to format parameter in discussion of statement serialization
Minor tidy up and amends

Note: I followed the heading sizes listed here:

https://github.com/adlnet/xAPI-Spec/issues/103
Fixed over zealous hashes #
change ID language to SHOULD and add to MUST and SHOULD example
add UUID RFC reference
add link to statement API
rephrase to avoid confusion over what the legal inverse functional identifiers are
remove objectType from actor in minimal example statement, as it is neither a MUST or SHOULD property in that location
clarify requirements for objectType in Activity objects using MUST language
specify ID instead of URI
Broadened reference to verb lists

Addresses https://github.com/adlnet/xAPI-Spec/issues/18
apply format "canonical" to activies only

* typo fixes
* applied "canonical" format to activities and not agents, since agents will not
have extensions, and there isn't a lot of point in normalizing name across statements.