Makefile.am: don't display pointless success banner
Makefile.am: add 'doc' target
Reference and require Singular >= 4.0.1
Do not install certain gap kernel handler funcs

Specifically, do not touch
 IsbbListFuncs
 UnbbListFuncs
 AssbListFuncs
They are not really being used by GAP for now, and they
are likely to be removed again in the future. Our
kernel ext is probably the only piece of code using
them right now, outside of the GAP kernel.
doc: allow building the manual when package is not in GAP root dir

By adding
  SetPackagePath("SingularInterface", ".");
to makedoc.g, the script becomes more robust and works correctly
even if SingularInterface is not currently in a pkg dir visible
to GAP.
Change version to 0.6.

Closes #21.