Fix for LOGBACK-875

Prevent logging on an interrupted thread from shutting down prudent file
appenders due to a FileLockInterruptedException
Merge pull request #218 from nemecec/master

Fix for Prudent FileAppender and thread interruptions (LOGBACK-875)
Add release note for LOGBACK-875 fix
fix LOGBACK-116

Correct RollingCalendarTest. Modify time-zone offset handling.