Defined a namespace DCF77 with time_t type and tm structs.

Adjusted some comments that were still mentioning the old library name an URL.
Defined a namespace DCF77 with time_t type and tm structs. DCF77::time_t and DCF77::tm will struct will be an alias for std::time_t and std::tm
in case that the platform supports std (e.g. on SAM platform). Otherwise it is an own definition (e.g. on Avr platform).
There are also now additional global utilitiy functions available under the DCF77 namespace: print_tm(), tm_to_timestamp() and
timestamp_to_tm().
Moved DCF77tm from subfolder 'internal' to main src folder.