This commit contains the first functional integration of Gaia Sky with [SAMP](http://www.ivoa.net/documents/SAMP/).
Since Gaia Sky only displays 3D positional information, and knows how to parse
stars, there are a few restrictions as to how the integration with SAMP is implemneted.

The current implementation only allows using Gaia Sky as a SAMP client. This means that
when Gaia Sky is started, it looks for a preexisting SAMP hub. If it is found, then
a connection is attempted. If it is not found, then Gaia Sky will attempt further
connections in the future using a specified interval of 10 seconds. Gaia Sky will
never run its own SAMP hub, so you will always need a SAMP-hub application (Topcat,
Aladin, etc.) to use the interoperability that SAMP offers.

Also, the only supported format is VOTable. There are, however, some restrictions
as to how the VOTables received through SAMP are interpreted and used (if at all)
by Gaia Sky:

- For the **positional data**, Gaia Sky will look for spherical and cartesian coordinates. In the
case of spherical coordinates, equatorial (`pos.eq.ra`, `pos.eq.dec`), galactic (`pos.galactic.lon`, `pos.galactic.lat`) and
ecliptic (`pos.ecliptic.lon`, `pos.ecliptic.lat`) are supported. To work out the distance, it looks for `pos.parallax` and
`pos.distance`. If either of those are found, they are used. Otherwise, a default parallax of 0.04 mas is used. With
respect to cartesian coordinates, it recognizes `pos.cartesian.x|y|z`, and they are interpreted in the equatorial
system by default.
If no UCDs are available, only equatorial coordinates (ra, dec) are supported, and they are looked up using
the column names.
- **Proper motions** are not yet supported via SAMP.
- **Magnituded** are supported using the `phot.mag` or `phot.mag;stat.mean` UCDs. Otherwise, they are
discovered using the column names `mag`, `bmag`, `gmag`, `phot_g_mean_mag`. If no magnitudes are found,
the default value of 15 is used.
- **Colors** are discovered using the `phot.color` UCD. If not present, the column names `b_v`, `v_i`,
`bp_rp`, `bp_g` and `g_rp` are used, if present. If no color is discovered at all, the default value of 0.656 is used.
- Other physical quantities (mass, flux, T_eff, radius, etc.) are not yet supported via SAMP.

This commit fixes #246