src/install.libs.R instead of 'configure' and inst/

instantiate 0.1.0

• Use install.libs.R for both CmdStan in instantiate and Stan model compilation in packages (#1, #9, #12).
• Stan models should now be in src/stan/ instead of inst/stan/ (#12). inst/stan/ should still work for the next few versions, but it is now deprecated. When a modeling package is installed, its install.libs.R moves src/stan/ to bin/stan/ and then compiles models inside bin/stan/.
• Disable staged installation for instantiate itself to help the hard-coded paths in CmdStan work as expected in "internal" installation.