Implementation of onEventIf, a version of onEvent guarded by confirm().

H/t for this little innovation actually belongs to @shadowfiend. I'm
just taking credit for bringing it into the latest version of Lift. ;)
Merge pull request #1450 from lift/msf_issue_1450

Implementation of conditional onEvent helper.
Null testing for headers. Closes #1452
Closes #1455. Finer grained sync
Fixed a real deadlock
Fix typo when constructing secured multi options
Bump version
Merge branch 'master' into ccap