Include all necessary files in "make dist" with automake

Previously only the files mentioned in Makefile.am were included,
which caused a tarball built with "make dist" to be unusable.

Now all files in the git repo are included (except for .gitignore
which really isn't of any use in a distribution tarball).
Merge pull request #7 from mstorsjo/automake-dist

Include all necessary files in "make dist" with automake