use strict_encode64 (instead of encode64) for user-data
remove the "in-place" reject (pretty useless)
adds two class method for Flavor:
 - find_by_constraints(constraints_hash)  => returns all flavors that meets the given constraints
 - applicable_for_image(image)                => returns all flavors that can be used for a given image
Version bump to 0.1.13
Regenerate gemspec for version 0.1.13
Merge pull request #7 from dguerri/master

adds two class method for Flavor