Update cmake_minimum_required to 2.8.3 for host/

Resolves issue #123

host/CMakeLists.txt uses CMAKE_CURRENT_LIST_DIR, which does not exist
before 2.8.3.  Updating this to avoid obscure build errors on old
machines.

Also adding a few missing cmake_minimum_required directives to avoid
a warning that likes to pop up when they're missing.
Add README note about minimum cmake version
Merge pull request #124 from rtucker/issue123_cmake_version

Bump cmake_minimum_required to appropriate value.