Add NewKeyPair for using an existing KeyPair.
fixes for httpjson changes.
Also update to use new errgo path.
gofmt
Merge pull request #23 from rogpeppe/025-update-for-jsonhttp-changes

fixes for httpjson changes.
Opinionated and specific bakery refactoring.
bakery: remove Discharger
Merge remote-tracking branch 'origin/cmars-003-caveatid-pkg' into cmars-003-caveatid-pkg

Conflicts:
	bakery/discharge.go
	bakery/keys.go
	bakery/service.go
	httpbakery/discharge.go