LOGBACK-960 Prevent a possible NPE in SyslogAppenderBase.close() when the SyslogOutputStream is null (due to a host lookup failure for example)

Link to the building doc fixed.

Added note for LOGBACK-959

Added note for LOGBACK-936, fixed by Eric Dahl in pull request #173

LOGBACK-936 Update SyslogAppender to format days according to the spec

This also updates the SyslogAppender to cache formatted time strings for the each second rather than each millisecond.

Use StringBuilder instead of StringBuffer in private code

The substitution parser attempted to deref a null pointer when a given
variable had a blank value. c.q.l.core.subst.Parser#T() was missing
a null-check.

This was discovered in logback-android's AndroidManifestPropertiesUtil.

LOGBACK-942 Update SocketAppender to wait up to a configurable amount of milliseconds before dropping a log event.

Add note for pull request #38 -- throw exception for unknown config
file extensions.