webfaf2: don't return url field in solution if no url is given by solution finder
pyfaf: find solutions by problems and opsysreleases
pyfaf: add probable fix solution finder
pyfaf: Solution class instead of namedtuple
pyfaf: better texts for probable fix solutions, since date

Closes #314
pyfaf: handle list of solutions when finding solution for a problem
pyfaf: link probable_fix to Build instead of saving nevr string
pyfaf: don't return a solution if affected package is newer than prob. fix

Closes #316
tests: add test for probable fix solution finder