Fallback on license field

On PyPI one can only use well-known licenses in the classifier fields.
The field 'license' is a usefull fallback to lookup the license name in
case no classifier is available.
Make license field the primary source

Only fallback to classifiers if it is unavailable or someone blobbed the
whole license text in this field.
Simplify get_license implementation slightly
Merge pull request #10 from coldfix/license-fallback

Fallback on license field