Added Edition::Less to allow std::container on Edition using

Added Edition::Less to allow std::container on Edition using
Edition::operator< as well as lexicogrophical order.

Namespace for string related tools and helper functions was
renamed from (zypp::)base::string to (zypp::)str. 'string'
is obviously not a good choice for a namespace, and the
additional base:: is superfluous and inconvenient.