Redefine "base URL" as "log URL" proposal

Background
---
The current api.md specification requires that a log has a fixed unique "base
URL".  It is any valid HTTP(S) URL that can end with "/sigsum/v0/<endpoint>".

Proposal
---
Remove the term "base URL" and instead define "log URL".  A log URL is a valid
HTTP(S) URL that ends with "/sigsum/v0/".  Example of a valid log URL:

	https://example.com:4711/opposum/sigsum/v0/

This means that a named sigsum endpoint can be appended to a log's URL.  For
example, if the endpoint is "get-tree-head-quickly" the resulting "endpoint URL"
would be:

	https://example.com:4711/opposum/sigsum/v0/get-tree-head-quickly/

And with input parameters for "get-leaves":

	https://example.com:4711/opposum/sigsum/v0/get-leaves/42/4711/

Note the final slash in all of the above URLs.  Should that be enforced (?).
	* XXX: Need to check in URL specification(s).  Defer for now.