Switch to the new get-pip.py URL

Add ``default-token-count`` configuration item

The redeemer will submit this many tokens with its redemption request now.  It
needs to agree with whatever the issuer wants to do, of course.

Introduce `direct_tahoe_configs` strategy

It builds _Configs instead of functions that return _Configs.

CircleCI-based macOS CI

Fixes: #99

Since I don't want to build another right now, drop it.

So optimistic...

Accept tahoe-lafs as an optional parameter

Fall back to our internal packaging if it is not supplied.

Also, shift towards less use of overlays.  Explicit arguments are better when
possible because they don't induce freaky action at a distance.

Make ShareStat actually copyable

Also add a test that actually serializes it

Add an HTTP API for price calculation