Implement fancy indexing for DOK matrices

This PR implements fancy indexing for DOK matrices.

I didn't yet benchmark the `__getitem__` logic in assume-sparse vs. assume-dense item fetching.
Also slicing is not benchmarked.

**EDIT**: [benchmark here](https://gist.github.com/pv/8635783/raw/a4d17640177373fdf53e950c6d4f42b71f66220a/cmp2.txt) --- speedups for N > 1, perhaps minor slowdown for N=1. As the slice optimizations were removed in getitem, there's a slowdown for low-density matrices.

**EDIT2**: [updated benchmark](https://gist.github.com/pv/8677247/raw/687b28b78d134658bf3eda8e396cd0bf4d66b5d5/cmp3.txt) adding the slice optimizations back. Nothing else to add AFAIK.