database package (of course not finished, GSOC isn't over yet)

I think it's now time for a pull request to ask for feedback, maybe I should have done that earlier. All tests pass and there is a coverage of 100%, but that doesn't mean there are no bugs of course.

My next plans:

- improve querying using the Wave attribute from VSO (i.e. search all entries and not only the ones with the given unit and convert if sensible)
- support the vso.attrs.Time attribute in the query method
- save a list of HDUs instead of one HDU per entry
- function display_entries: support pretty output of FITS header entries
- support adding entries by HEK result(s) (I think I will use the HEK2VSO package and especially its function ``translate_results_to_query``. Any thoughts or opinions on that?)

Finally, there is still a database guide for the documentation missing. But at least I have written the headlines on a local file :)