Updated client lib spec to v1.2

[tomczoink]

• Updates the client lib spec to v1.2, adding in a diff and v1.1 folder
• Adds seperate versioning for client lib folder and doc folders, so as to allow the version of the spec to change prior to the version of the rest of the docs. This can be easily extended to other folders in the future if needed

The 1.2 spec changes have been taken from #843.

[SimonWoolf]

LGTM, except for the same comment I made here: the 1.1->1.2 diff is not right, looks like it's from 1.1-as-amended to 1.2 when it should be from 1.1 to 1.2 - that is, it should include all the changes that have been added to the 1.1 spec since its initial release, which are required for 1.2 compliance per https://github.com/ably/docs/#branch-and-tag-scheme-for-features-spec.

(To put it another way: it should be the case that if you start with the 0.8 spec, and apply the 0.8->1.0, 1.0->1.1, and 1.1->1.2 diffs, you get to the 1.2 spec, which meaqns that the endpoint of the last diff needs to be the startpoint of the next one)