internal/charmstore: make stats counter tests less hacky.

internal/charmstore: move mongojs condition into storetesting.

internal/router: universal ParseForm.

internal/router: strip prefix from global handlers

internal/router: add ServeMux, NotFoundHandler
internal/v4: minor changes for review
Merge pull request #52 from rogpeppe/023-v4-stats

internal/v4: implement stats endpoint

manual merge because 'bot is down.