Add mapping from status type to human readable text

[mfeurer]

Currently, the status is an integer between 1 and 5. It cannot be easily displayed to the user without explaining it. It would be helpful to add a mapping back to a human readable string, so that I can do:

>>> print(mapping[status])
Memout

In the course of implementing this, we could also start using the Enum class which was added in python3.4.

[mlindauer]

Yes, very good idea to use Enum

[shukon]

Enum affects the JSON-serialization. It is implemented in #151, using a customized JSON-Encoder. I assume, that the JSON-files are only ever read in by smac (more precise: the runhistory)?
mapping is realized by "status.name", which returns the status in human-readable form (-> "SUCCESS").