Add facility to Syslog::Logger

Hi All,

This pull request about adding the facility level to Syslog::Logger without closing and reopening syslog just using priority from syslog(3)

This allows have different instances of Syslog::Logger which will log using different facility level. Example:
```ruby
require 'syslog/logger'

user_log = Syslog::Logger.new "ruby"
local0_log = Syslog::Logger.new "ruby", Syslog::LOG_LOCAL0
local7_log = Syslog::Logger.new "ruby"
local7_log.facility = Syslog::LOG_LOCAL7

user_log.info 'this line logged using user facility'
local0_log.info 'this line logged using local0 facility'
local7_log.info 'this line logged using local7 facility'

```