Support tables in high-level API

[kbarbary]

This isn't finished yet, but I'm filing this now to let people know I'm working on it. This is ready to go.

You can already write data to a new table extension and read columns back. Demo:

julia> using FITSIO

julia> data = ["col1"=>[1., 2., 3.], "col2"=>[1, 2, 3]];

julia> f = FITS("tmp.fits", "w");

julia> write(f, data)  # write data to new extension

julia> f[2]
file: tmp.fits
extension: 2
type: BINARY TABLE
rows: 3
columns:
    col2 (1K)
    col1 (1D)

julia> read(f[2], "col1")  # read column 1
3-element Array{Float64,1}:
 1.0
 2.0
 3.0

I have a bunch of remaining to-do items, particularly testing and documentation.

[kbarbary]

• ASCII tables are now also supported and there are a few basic tests for both binary and ASCII tables.
• Documentation updated to reflect table operations and API documentation completed.

[kbarbary]

Rebased.

[kbarbary]

If there aren't any comments I'll merge this tomorrow.