(#2462) - Fix API documentation for cache constructor option

Updated the documentation for new PouchDB's cache option.
The flag is a sub property of the ajax option, not a top
level option.
(#2207) - provide additional information about the db.compact() method

Give some high-level information about the compaction operation itself,
highlight its availability only with the http adapter. Provide a link
to CouchDB's docs for more low-level detail.
(#2458) - explicitly mark idb changes as readonly

I had to Google for awhile before I figured out that
the default is readonly. This is the only case where
we're not being explicit.
(#2424) - add cors credentials to errors.md

Forgot to add this when I originally wrote this
error page. Not every user will use credentials,
but it's probably better to just have them enable
everything and then tweak it later after it works.
(#2447) - link pouchdb-server's pouchdb to us in travis

Travis seems to be an unusual environment where the
immediate dependents of a module that depend on that module
are not automatically symlinked to it.  So this fix is to
only do it in Travis.

This is required to unblock #2442.