added another channel `/dev/script`

some Python users were unhappy with no dedicated channel for scripts
added `/dev/script` as a dedicated script channel
the default co-location logic was adjusted to not use `script` channel as a co-located object
if you still want it to be co-located for some strange reason you can use `"attach": "script"`

P.S. I'm considering to use `"attach": "none"` as a default behavior for nodes with just a `script` channel
but let's wait and see
Merge pull request #80 from pkit/script

added another channel `/dev/script`
produce better error codes

if one of the queries in clustered environment failed, the final response was still "200 OK"
now the server will choose `max(all_error_codes)` as a final error code and return it to user instead of "200 OK"
correct error messages for each particular task (if any) can still be parsed from `X-Nexe-Error` response header
Merge pull request #81 from pkit/error_codes

produce better error codes
refactor channel mangling in config parser

the function `_add_new_channel` was too opaque and created things under the hood
the function `_get_new_node` was mutable
now it should be clear what's going on just from looking into the `parse()` flow
fix nodes with only networked channels

the nodes that had only network channels attached would not show in final config
it lead to various strange errors, like `zvm://` urls not being resolvable