Added CategoryErrorLevelActivationStrategy, which enables setting the
Error Level threshold per log channel.
Refactored / Renamed the CategoryErrorLevelActivationStrategy to the
more appropriate ChannelLevelActivationStrategy.
Fixed phpdoc, typo and indentation issues.
add error_log handler

[error_log](http://www.php.net/manual/en/function.error-log.php) handler
to send output to webserver SAPI handler (directly or indirectly)
Merge remote-tracking branch 'glensc/errorlog'
Add new handler to the README
Merge remote-tracking branch 'netmikey/master'
Fix a few details and add docs to the ChannelLevelActivationStrategy, refs #186