Initial commit for the ESO package.

For now only authentication on the ESO portal works.
Bonus feature: keyring!
Needs a maybe new dependency: beautifulsoup 4
Use lxml rather than BeautifulSoup

Seems more standard that way…
Added lxml and keyring to package requirements
Added lxml and keyring to .travis.yml
Added method to query a specific instrument
Make sure that keywords are strings when filling the form.
First implementation of a generic _activate_form method

Not complete yet.
Use _activate_form() in login() method.
Clean-up login() method to call authenticate only once.
Simplified query_instrument() using _activate_form().
Merged authenticate() into login()
Fixed missing '/'
Improved handling of file upload with forms.
Made not implemented enctype-dependent post more verbose.
Implemented handling of 'application/x-www-form-urlencoded'
Converted form_dict from dictionary to list. Added handling for CheckboGroup.
Initial implementation of a direct data_retrieval() method.

For now, it only returns the response object of the data download page.
Work in progress…
Brand new implementation of _activate_form()
Removed unused import: re
Added _download_file(self, url) to Eso class.