Week 2

This week, I worked on the backbone of the indexing system I'll be working on--I have an Index class that will be compatible with a number of data structure implementations, and for now a binary search tree implementation. See https://github.com/mdmueller/astropy/blob/table-indexing/astropy/table/index.py and the implementation, https://github.com/mdmueller/astropy/blob/table-indexing/astropy/table/bst.py. I also have tests to make sure that the BST structure works as expected. (Nodes of the BST are of the form (column value, row number) for now, although this will probably change to something like ((column 1 value, ..., column n value), [row 1, ..., row m]) later to deal with composite indices and repeated column values across multiple rows.)

Next week I'll be integrating the new Index class into the existing functionality of the Table class, as well as implementing composite indices (indices on multiple columns) as well as a new `where()` function to find rows by column values.