Adding default_rng_t type to use in place of mt19937 for better
modularity (and its easier to remember when coding)
Merge branch 'master' of https://github.com/ksimek/libergo
Merging changes from main repository.

Conflicts:
	ergo/hmc.h
	ergo/mh.h
Two big refactors:

1. Added backward compatibility for older boost::random API by
    introducing wrapper objects for random number generation.

2. Replaced multiple constructor overloads with a single one by using
    the new "copy_or_ref" class, which allows calling context to
    choose between external, internal, or shared ownership semantics.