Fix slicing and implement some pandas methods

Starting on #19. This PR gets slicing working for `GeoSeries`, as well as a number of other pandas methods ([`head`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.head.html#pandas.Series.head), [`tail`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.tail.html#pandas.Series.tail), [`order`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.order.html#pandas.Series.order), [`sort_index`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.sort_index.html#pandas.Series.sort_index), [`take`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.take.html#pandas.Series.take), and [`select`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.select.html#pandas.Series.select), at least).

Fixing `.loc`, `.iloc`, and `.groupby` are rather more involved, and this PR does not attempt to start implementing them.  There are tests for those in this PR, currently explicitly skipped.