nitg: Added PNaCl support for Nit

Added pnacl_platform.nit in order to generate all minimum files when compiling the nit.
Added pnacl.nit, it's the module providing the tools for PNaCl support in Nit.

Modified nitg.nit to register the pnacl_platform.
Modified abstract_compiler.nit for a specific main generation and to add an option.

Signed-off-by: Djomanix <johan.kayser@viacesi.fr>