Snap-base

- Timing module fix (now absolute timing instead of relative by default)
- Exceptions now cause the process to stop

Timing bug fixed

* ZMQ status report server available
* `.timing.every` utility element added
* `.output.dump` element output can now be rewritable
* Documentation update


v.1.3.0
Reorganized modules structure
Now all steps/sources are under `snap.elements` module
Default module in the configuration file is now `snap.elements`, not `snap`

* Added hopskotch integration
* Moved the zmq/hop interfaces to a separate "io" module
* Default install "pip install snap-base" contains only core functionality, the io is in plugins.
* Added documentation in RTD

* Handling `SIGTERM` and `SIGINT` to stop current tasks and exit gracefully.
* Exceptions during configuration parsing are more informative.
* Better logging output
* Snap verbosity flag `-v` now accepts the logLevels: `DEBUG| INFO | WARNING | ERROR`