Move find_features method into NucleotideSequence

This is about what I had in mind for #795.
Can you please list the acceptable feature types here? This will resolve #746.

[numpydoc](https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt) recommends using the following formatting:

```
feature_type : {'purine_run', 'pyrimidine_run'}
```