prepare for freeform REST services

this patch generalizes `version` part of url into REST interface version
it also adds ability to place version in `X-Zerovm-Execute` header

Example:

`GET /open/account/container/obj` == `GET /v1/account/container/obj` + `X-Zerovm-Execute: open/1.0`
Merge pull request #98 from pkit/open

prepare for freeform REST services