add support for MSVC setup script rewrite

MSVC setup scripts are very slow, so should not be used in build actions directly.
Rewrite these scripts into their fast equivalents and use those instead.
Add toolset initialization option <rewrite-setup-script> to customize/disable this new behavior.
Merge pull request #16 from jivancic/slow_msvc_setup_scripts_2

add support for MSVC setup script rewrite