Have *Logger.Flush() not return nils in the []error slice it returns.
If there are no errors, then it returns an empty slice.
Change RollingFileAppender to be synchronous, in preparation for
factoring out the asynchronous part into AsyncAppender, and having a
new AsyncRollingFileAppender that composes the two.
pull up rolling_file_appender.simpleLog() into slogger.SimpleLog()
Add AsyncAppender
Fix header comment in async_appender.go to refer to async_appender
move assertNoErrors to new test_util package
Fix async_appender.TestConcurrentLog to wait for logging goroutines to
finish.

Add some AssertNoErrors() checks to async_appender_test