Initial impl. of receive based hooks.
Store repo_path inside hooks.

- change some builtin parameters names
- report stdout back to client for pre-receive hook
move re import to global scope
Introduced concept of pre/post upload hooks
improved some docs
Let the shell hooks do seralization of data.

We mimic stdin only in the shellhook itself, if we use pure-python hooks then we pass original list of refs
Some code cleanups in hooks execution, including that shellhook always return the ret code