enabled possiblity to specify the checkout for any given dependency.

@petya2164, @mortenvp. We can now checkout the master, commit id, or brach of any dependency. This will enable the master branch of the repositories to always be releasable.

The option is specified from the commandline as follows:

./waf configure --bundle=ALL --bundle-path=../dependencies --fifi-use-checkout=boost_to_std