More sensible signature functions

Remove all of the older functions. Bind git_signature_default() and
implement the _now() equivalent in erlang.

c_src/signature.h

@@ -1,3 +1,4 @@
 #include "geef.h"
 
 ERL_NIF_TERM geef_signature_new(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[]);
+ERL_NIF_TERM geef_signature_default(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[]);