Implement windows event logging

[ColinSullivan1]

Currently, the windows sys logger is a stub for standard logging. This PR implements windows event logging as the “syslog" for the NATS server on windows.

Using the -s or -r parameters on windows will enable logging to the windows event log.

• The event source is NATS-Server, and data is written to the Application log.
• Writing to the event log requires an elevated privilege. If the user does not have required privileges, the server panics.
• Log messages are written to the Application event log in the following format: pid[<pid>]:[<type>]: Message
• Notice, Trace, and Debug messages are written as info messages and Errorf or Fatalf log messages are logged as event log errors.
• Remote logging is supported.
• Includes tests and vendoring

Here is what a NATS server log entry looks like in the windows event viewer:

[coveralls]

Coverage decreased (-0.3%) to 89.217% when pulling 1df5af0 on windows-event-log into bc5f864 on master.