improve zapp format

according to [zerocloud/doc/Requests.md](https://github.com/zerovm/zerocloud/blob/swift-2.0/doc/Requests.md#post-a-zerovm-image-file)
the tar image should include `boot/system.map` file to become executable in Zerocloud

renaming `<zapp name>.json` to `boot/system.map` solves that incompatibility
it can be used to directly execute zapps on zerocloud clusters without deploying them first
fix _post_job

_post_job used `Accept` header which is not needed
it also used unconfigurable `Content-Type` header, although jobs can be of various content types
add option to execute zapp without deploy

now `execute` command has an optional `--container` argument, rather than required
if `zpm execute zapp_path.zapp` is invoked the zapp will be executed directly without deploying
if `zpm deploy zapp_path.zapp --execute` is invoked zapp will be deployed and then executed
if `zpm execute zapp_path.zapp --container container_name` is invoked an already deployed zapp will be executed

I think the last one should become `zpm execute swift://./container_name/zapp_name.zapp` instead, but lets leave it for now
Merge pull request #163 from pkit/direct_execute

Add functionality to directly execute on ZeroCloud (without deploying)