Table with generalized columns

Triggered by #1833, I thought I would also put the implementation @wkerzendorf and I have been working on for making generalized columns, which can hold any type of array.  This is by no means finished, but as our approach is quite different, I thought it was useful to put it out there.

In the `Column` class, the addition to properly support `Quantity` and its sub-classes is remarkably trivial; one just needs to keep track of the few attributes that are needed beyond just the `__class__` (`_unit` for `Quantity`; also `_wrap_angle` for `Longitude`; can just get these from `__dict__`).

For things other than `ndarray` subclasses, the main concept is that anything can become a column as long as it can turn itself into a properly sequenced array. We tried only for `Time` where one turns either just `jd1, jd2` into a 2-item array (to be made structured), and stores all relevant attributes in a dictionary (e.g., `lon`, `lat`). If any of the attributes are also arrays, they get stored in the full array as well (e.g., `delta_ut1_utc1`, or indeed `lon` and `lat`). The class can take a `Time` view of this by letting `jd1`, etc., point to the appropriate parts of the combined array.

Note that in this implementation, `Column` no longer is a subclass of `ndarray` itself; it seemed essentially impossible to have it be that, and have things like `np.sum(table['q'])` return a `Quantity` if the corresponding column holds a `Quantity`. One advantage is that this holds the promise of just storing a `MaskedArray` in a `Column`, thus obliviating the need for `MaskedColumn`, or indeed having whole tables be masked when really only a single column needs to be.

Anyway, maybe best to give an example of what's possible now (ignoring that many test cases are now broken...):
```
In [1]: from astropy.time import Time, TimeDelta; from astropy.table import Table, Column; from astropy.coordinates import Angle, Longitude, Latitude; import numpy as np; import astropy.units as u

In [2]: t = Time(['2012-01-01', '2013-01-01'], scale='utc')

In [3]: l = Longitude(np.arange(10.,12.), 'deg')

In [4]: q = np.arange(2.)*u.m

In [5]: b = Table([q,t,l], names=['q','t','l'])

In [6]: b['t'], b['q'], b['l']
Out[6]: 
(<Column name='t' unit=None format=None description=None>
<Time object: scale='utc' format='iso' value=['2012-01-01 00:00:00.000' '2013-01-01 00:00:00.000']>,
 <Column name='q' unit='m' format=None description=None>
<Quantity [ 0., 1.] m>,
 <Column name='l' unit='deg' format=None description=None>
<Longitude [ 10., 11.] deg>)

In [7]: b['q'].to(u.cm)
Out[7]: <Quantity [   0., 100.] cm>

In [8]: b['t'].tdb
Out[8]: <Time object: scale='tdb' format='iso' value=['2012-01-01 00:01:06.184' '2013-01-01 00:01:07.184']>

In [9]: b['t'] + 1.*u.min
Out[9]: <Time object: scale='utc' format='iso' value=['2012-01-01 00:01:00.000' '2013-01-01 00:01:00.000']>

In [10]: b['q'] += 1.*u.cm

In [11]: b['q']
Out[11]: 
<Column name='q' unit='m' format=None description=None>
<Quantity [ 0.01, 1.01] m>
```