Some big changes for 0.13

[saibotsivad]

Minor changes:

• Some spelling and minor document fixes by @TehShrike
• Correction to 0.12 private key size (was 2056, should have been 2048)
• Fixed a handful of broken links.

The 0.13 specs were created as a direct copy of 0.12 and has the following significant changes:

• The private_key schema no longer requires the fingerprint property, as the fingerprint requires an identity container to be created, and I didn't want to tie them together so much.
• The version number now includes only major and minor. Patches are not included in the version numbering.
• As part of #51 the container specs were greatly reworked. Container extension data is now held in properties named the same as the value in the sdmp.schemas list.

[saibotsivad]

This is a botched attempt at straightening out my local computer with Github. I'll repoen this pull request eventually here...