Swap the order of arguments in Column from name, data, .. to data, name, ..

[taldcroft]

Table Column and MaskedColumn will now have the initializer signature

def __new__(data=None, name=None, ...)

This was discussed in #726 and then made possible as a smooth transition by #731. In particular #731 (which is in 0.2) issues a warning if users call Column with an initial column name arg value instead of using keyword args.

In this PR there is some compatibility helper code. If the user supplies an initial arg value which is a string (corresponding to the pre-0.2 method), this raises an exception with a useful message:

>>> Column('a', [1, 2])
...
ValueError: 
The first argument to Column is the string 'a', which was probably intended
as the column name.  Starting in Astropy 0.3 the argument order for initializing
a Column object is Column(data=None, name=None, ...).

[taldcroft]

Can someone have a look at this?

I'd like to get this in soon because the doc updates for #726 depend on it, so this is blocking progress there.

[astrofrog]

@taldcroft - this looks good to me! One minor comment, maybe in generate_ref_ast the arguments should also be switched around, and data= removed? (as in other places in the code?)

[taldcroft]

With the changes in #726, the syntax in generate_ref_ast will be obsolete anyway. All those would be better written like t['obstime'] = obstime, so I'm inclined to just leave generate_ref_ast.py alone for now. My guideline has been that documentation must use the preferred syntax, but test code only has the requirement of working.

[astrofrog]

@taldcroft - sounds good - in that case, feel free to merge. Looking forward to the awesome new syntax!