Add install target to Makefile

Installs everything under `DESTDIR', which is `/usr/local' by
default.
Add more documentation formats to Makefile

`dvi', `pdf' and `ps' can be compiled/installed by explicitly
specifying `make FORMAT' or `make install-FORMAT'.
Add uninstall target

Removes all files from their installed locations.
Use more variables

This allows any command used to be overridden, when, for example,
they're not installed in PATH.
Add install-strip target