Clarifications on fragids. See #50.

[ioggstream]

This PR

• includes clarification on JSON Pointer and alias nodes compatibility
• includes FAQ
• fragment identifier moved from media type to section
• empty fragment + / are both under JSON Pointer

[ioggstream]

cc: @cabo @handrews PTAL

[ioggstream]

According to @handrews this PR should better clarify JSON Pointer use, so I am going to merge it.

The decision on forcing +yaml fragment is deferred to #54 that I will rebase.