Range::size() returned value type

Range::size() returned value type

This patch allow the user of the Range template class to define the type of
the returned value of Range::size() method, adding a second template argument.
The default type of the return value of Range::size() method is size_t, which
in most cases is enough (but not always, eg  HttpRange).
This patch will allow Range template to be used with non numeric types.