Make the "lib" target depend on the "libntru.so" target

Depending on the old "lib" target caused a rebuild of libntru.so on any
such target (e.g. installation), because "lib" is in .PHONY
and not an expected file name.

Fixing this helps with distro policies which require to not do
compilation processes during installation phase.
Allow building and installing static lib

Refactored the install rules a bit, so that installing the static
lib optionally becomes a bit more convenient.
Update VERSION in Makefiles
Install docs in version-specific directory by default

This is standard in most linux distros.
Add libntru.a to "clean" target
Merge pull request #12 from hasufell/master

Make the "lib" target depend on the "libntru.so" target