Add discard border method to FilterObjects

Externalizes the method to discard border objects used in
IdentifyPrimaryObjects by adding it to FilterObjects. Useful if one
wants to use ReassignObjectNumbers to unify objects which were
improperly split prior to discarding them.
IdentifyTertiaryObjects: Option to not shrink

Implemented option to not shrink the smaller primary object before
subtracting the two objects.
IdentifyTertiaryObjects: Test added

Added test for the option to not shrink the smaller (primary) object.
Test for the FilterObjects discarding mode

And refactoring of the ROM_ literals to MODE_, and the
module.rules_or_measurements property to module.mode.
Merge branch 'push_tertiary' of git://github.com/BioTurboNick/CellProfiler into issue_427_428_review
Merge branch 'push_discard' of git://github.com/BioTurboNick/CellProfiler into issue_427_428_review
Minor change to upgrade_settings: create new settings_value list instead of appending to old to avoid modifying the input list
Added test case for objects touching the edge of their parent image's mask + fixes to filter objects for that code path
Associate primary objects of unshrunken tertiary objects + corner cases for disappearing unshrunken
Merge branch 'issue_427_428_review'