Use [ ] quotes in AC_PREREQ
Change default package version to 0.1

Using 0.1dev (which means "version infinity") has too much potential
for causing confusing problems.
Add support for C++ kernel extensions

Closes #6.
Do not output empty address fields in PackageInfo.g

These trigger an error in GAPDoc when running makedoc.g
Enable questions about repository and GitHub

The code for handling them is still WIP, though.