added the missing define for radio_packet_length_t on econotag
update Kaspar's email address

kaspar.schleiser@fu-berlin.de is obsolete.
(2nd try, first try was overwritten by some overzealous documenter)
Merge pull request #552 from OlegHahm/radio_packet_length_t_for_econotag

added the missing define for radio_packet_length_t on econotag
Merge pull request #574 from kaspar030/kaspar_email_change_2

update Kaspar's email address