update API: include all SRP/stretching parameters, change salt names

If we update the code to implement these APIs, we'll be able to deliver
all the stretching/salting information the client needs. The current API
spec is too limited in some places (only delivering one number for
stretching, when actually we need 5 plus a type, and overspecified in
others (naming a hash algorithm and group-size-in-bits for SRP, when
really it should just be a single name for the whole parameter set).

I'd recommend calling the SRP salt "srpSalt" rather than "s". There is a
different SRP object named (capital) "S", which absolutely must be kept
secret (it is the derived shared key), and using the same letter for
both is bound to confuse somebody down the line into publishing "S" by
mistake, which would be disastrous.

Feel free to fix the spelling (e.g. scryptN instead of scrypt_N), I
don't know what the local conventions are.
Rewrite API docs to match 31-Jul-2013 design.
docs/api.md: update internal anchor links
api.md: /password/change also returns a keyFetchToken

We want this to grab kB before submitting an account reset.
api.md: rename /password/change to /password/change/start
Minor api.md changes

* fixed json typos
* changed /account/delete to /account/destroy
* fixed error example