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.

Throw exception for unexpected file extension for config

Add note for pull request #132 (LOGBACK-269)

Add support for appenderRef in Groovy configuration

Ensure that exception is thrown if configuration file has unexpected extension (not .xml or .groovy)

Patched up note on dangling database connections

Always close jdbc Statement/ResultSet in finally