Week 10

This week wasn't terribly eventful; I spent time documenting code, expanding tests, etc. for the pull request. Docstrings are now in numpydoc format, and I fixed a few bugs including one that Tom noticed when taking a slice of a slice:
```
from astropy import table
from astropy.table import table_helpers

t = table_helpers.simple_table(10)
t.add_index('a')
t2 = t[1:]
t3 = t2[1:]

print(t3.indices[0])
```
The former output was "Index slice (2, 10, 2) of [[ 1 2 3 4 5 6 7 8 9 10], [0 1 2 3 4 5 6 7 8 9]]" while now the step size is 1, as it should be. The SlicedIndex system seems to be working fine otherwise, except for a python3 bug I found involving the new behavior of the / operator (i.e. it returns a float), though this is fixed now.

Another new change is to the `index_mode` context manager--the "copy_on_getitem" mode now properly affects only the supplied table rather than tampering with BaseColumn directly. Michael's workaround is to change the __class__ attribute of each relevant column to a subclass (either _GetitemColumn or _GetitemMaskedColumn) with the correct __getitem__ method, and this should rule out possible unlikely side effects. Aside from this, I've also been looking into improving the performance of the engines other than SortedArray. The main issue I see is that there's a lot of Python object creation in the engine initialization, which unfortunately seems to be unavoidable given the constraints of the bintrees library. The success of SortedArray really lies in the fact that it deals with numpy arrays, so I'm looking into creating an ndarray-based binary search tree.