Add a target for installing libexru and exdemo.

Previous commits mixed both an installed and relocatable
demo together. This was quite confusing, so the
relocatable demo has been moved to the 'relocatable' branch.
Additionally, use 1.0.0 of exru.
Also tidy up the repo with an install script and more docs.
Use 1.0.0 of exru
Merge branch 'master' into relocatable

Relocatable is just a variant of master to
demonstrate a different cmakelists.  So just
merge it in.

Conflicts:
	CMakeLists.txt