Remove BookSite recipe and Velocity theme

[daveaglick]

Heads up: the BookSite recipe and it's Velocity theme are going to be removed in an upcoming release (probably the release after next).

I realize this stinks for sites using that recipe, but all the recipes will eventually be merged in v3 (see #668) and while Blog and Docs have enough in common to make merging them fairly easy, BookSite is totally different and I don't see it surviving the merge. Because of that, when combined with the much lower usage numbers, I just don't see the value in spending time continuing to maintain it. If anyone really needs to continue using BookSite, I'd suggest you stay on the most recent version of Wyam that has it (probably 2.1.2), download the code from that release and continue packaging BookSite manually, or port the BookSite code back into a Wyam config file.