Psf analytical class

This implements a first version of `EnergyDependentMultiGaussPSF` and `EnergyDependentTableARF` classes. Test data and a corresponding `gp-irf-info` script is also contained.
In library code such as `gammapy` you shouldn't configure the logger ... that should be done by the end-user.

There's some info here on how to use logging and warnings in Astropy and we should use the same in gammapy (I think):
http://astropy.readthedocs.org/en/latest/development/codeguide.html#standard-output-warnings-and-errors

Concretely to log some info do this:
```
from astropy import log
log.info('info')
```
To warn about something:
```
import warnings
warnings.warn('warning')
```
I haven't thought about this yet if this will work nicely for users, but that's the recommendation from Astropy, so probably yes.