Introduce argument -l/--log-level to set the logging level
Add a PyOP2 logger and set log level according to configuration
Add common logging functions debug, info, warning, error, critical
Factor out MPIConfig into separate module
Factor logger out to separate module
Define default log level in default.yaml
Use op2.warning instead of warnings.warn in backends
Use op2.debug in petsc_base
Merge pull request #180 from OP2/logging

Add proper support for logging levels compatible to the logging
module from the Python standard library.

* add command line switches -l/--log-level to utils.parser
* add a logger module controlling the PyOP2 logger
* add functions debug, info, warning, error, critical to the op2 namespace
* factor out MPIConfig to its own mpi module
* replace print calls in the pyop2 library