Only use install_name for shared libraries

This is an OSX-specific fix.
Automatically add -soname to libraries

This way I don't have to manually set SONAMEs for each of my
libraries.  This is really an OSX-specific fix because OSX has a
different mechanism for naming shared libraries.

I'm going to call this a backwards-incompatible change because it does
make a difference!
Bump version to 0.6.0