Relative hyperbolicity and relative quasiconvexity for countable groups

We lay the foundations for the study of relatively quasiconvex subgroups of relatively hyperbolic groups. These foundations require that we first work out a coherent theory of countable relatively hyperbolic groups (not necessarily finitely generated). We prove the equivalence of Gromov, Osin and Bowditch's definitions of relative hyperbolicity for countable groups.