Include an optional "choices" atribute

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/17)) by Carlos Agüero (Bitbucket: [caguero](https://bitbucket.org/%7Bda8a8e89-4bb0-421b-bd0e-dbbed3d4ed6a%7D/), GitHub: [caguero](https://github.com/caguero)).**

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Something like this:

```
#!c++

The type of noise. Currently supported types are:
"none" (no noise).
"gaussian" (draw noise values independently for each measurement from a Gaussian distribution).
"gaussian_quantized" ("gaussian" plus quantization of outputs (ie. rounding))

```

Inspired by the *choices* keyword used in [argparse](http://docs.python.org/dev/library/argparse.html#choices).