Issue #290:

add new API methods:

- Metacello>>onConflictsUseIncoming
- Metacello>>onConflictsUseLoaded
- Metacello>>onConflictUseIncoming:useLoaded:
- Metacello>>onDowngradeUseIncoming
- Metacello>>onDowngradeUseIncoming:
- Metacello>>onUpgradeUseLoaded
- Metacello>>onUpgradeUseLoaded:
- Metacello>>onWarningLog
- MetacelloResolveProjectUpgrade>>useIncoming
- MetacelloResolveProjectUpgrade>>useLoaded
Merge branch 'dev' of github.com:dalehenrich/metacello-work into dev
Issue #290: should be Metacello>>onConflictUseIncoming and Metacello>>onConflictUseLoaded
Issue #290: add test cases