-almost fixed graph generation, cycles still to do
-finished rewrite of solver graph generation, should fix bug related to PR #120
-added graph prune configurable option
-added support for literal and expandable strings in rex
-shell plugins now provide string expansion methods
-fixed remaining bugs wrt rex literal strings and quote escaping
-added unit test in test_shells
-upped internal context serialise version from 3.1 -> 3.2, reflecting the change in the resolve graph
-fixed bug where escaped quotes in old commands were not taken into account
Merge branch 'fix_solve_graph' into 2.0

Conflicts:
	src/rez/_version.py