Update Build Method on Build System to Use BuildType enum.

This pull request addresses issue #102.

Rez 2 was not exporting the `CENTRAL` CMake variable.  This is now supported in Rez 1 backwards compatibility mode.

A new CMake variable `REZ_BUILD_TYPE` is always exported as a longer term replacement for `CENTRAL`.  In the backend, this build type is made available for all build systems to use.

The `REZ_IN_REZ_RELEASE` environment variable is now also supported by the CMake build system only when in Rez 1 backwards compatibility mode.