Rename tool to drop extension

[mbrukman]

Currently, autogen.sh has the .sh extension as it is a shell script. In the future, we may want to rewrite it in a more powerful language, whether a scripting one such as Python or Ruby, or a compiled language such as Go.

In that case, everyone will have to update all of their pointers, and we will have to update all of our editor integrations, such as the one described in issue #13.

To avoid breaking existing users who upgrade in the short-term, we can implement this as follows:

• move the content of autogen.sh to autogen
• keep autogen.sh around for backwards-compatibility, simply as a wrapper for autogen