Python feature: config file to present Arbor's support on mpi, gpu and version (#708)

Fixes #700

Usage in python as: 

```
>>>import arbor
>>>arbor.config()
{'mpi': True, 'mpi4py': True, 'gpu': False, 'version': '0.1.1-dev'}
>>>d = arbor.config()
>>>arbor.print_config(d)
Arbor's configuration:
     mpi    :       True
     mpi4py :       True
     gpu    :      False
     version:  0.1.1-dev
```