added REST interface for POST

first REST interface support added, yay!
we now support POSTing stuff to arbitrary zapp archives
using `open` version or proper `X-Zerovm-Execute` header you can POST stuff into existing zapp
example:

	POST /account/container/my_zapp-0.1/my_zapp.zapp

the zapp should expect to get a POST request, which means:
- it should have { "device": "stdin" } without a "path" property, it will recieve the payload there
- it should have some other meaninful channels configured, for example { "device": "stdout" }
- it can use job-chaining to launch another sub-job
- it can use channels connected to containers to query the containers and create dynamic sub-jobs
- etc. etc.

there are still some caveats that will be probably sdressed soon, for example: the job-chained zapp cannot recieve the POST payload instead of API endpoint zapp