disallowed referencing variables from inside secondary index functions (c.f. Issue #718)
made `between` abort earlier if its left key is greater than its right key (c.f. Issue #614)
Removed MCHECK from src/build.mk, errors.hpp/cc.
Removed MCHECK from mk/way/default.mk.
Improved NaN Checking.
Improved groupby error message (c.f. Issue #525).
updated yaml docs to say aggregator
Merge branch 'next' into sam_etc_no_mcheck
added comment and changed to private inheritance for special_var_shadower_t
Merge branch '614_make_tests_run_in_debug' into next
merge fix
Merge branch '767_better_nan_checking' into next
Merge branch '525_groupby_error' into next
fixed merge error
Fixing accessing of invalid iterator in progress_app
Merge branch 'next' into sam_etc_no_mcheck
Merge branch 'next' into sam_etc_deheaderize

Conflicts:
	src/rdb_protocol/terms/rewrites.hpp
	src/rdb_protocol/terms/seq.hpp
Renamed fill_in_backtraces to preprocess_term.
Renamed propagate_backtraces to propagate_backtrace.
Moved rdb_protocol/terms/... headers to terms.hpp.