Add old features

[lgrcia]

Now that package has been created, here are some features to add:

• Loading camera from file
• Saving images as FITS
• Choosing different Gaia filter
• Relying on local database for Gaia query
• Add tests to continuous integration
• Separate Telescope vs. Camera
• Add a Conditions object that contains seeing, sky bkg, scintillation ... etc
• Public API?

[ppp-one]

I have added Telescope and Site. For site, we miss to add noise from scintillation and sun light scattering for flats.

The rest of the to dos I haven't touched, yet...