Fixed checkbox handling

Reject when not checked!
Added debug statement for form payload
Updated download link
Allow login() with no username

You will be asked for it…
Added test for retrieve_data()

MIDI is my favorite of the moment, sorry AMBER!
If no username specified, use configured one.
Marked the retrieve_data test as fail for now
Typo...
Updated CHANGES.
Add authenticated() to QueryWithLogin
Force login() is needed in Eso.retrieve_data()
Updated Eso documentation for automatic login/password
Doc: Small typo
Updated CHANGES
Merge pull request #420 from jwoillez/form_activation

ESO: fixes to retrieve_data() and improvements in login()
allow UKIDSS _check to work when logged in & add a more flexible
database identification tool
add changelog entry