Merge pull request #157 from VoiDeD/getevent-defaults

Expose optional default values for the GetEvent* SP functions (r=psychonic)..
Expose some of ICommandLine to plugins.
Implement GetServerSteamID native on engine branches that support it.
Implement GetServerAccountID.
Merge branch 'sp-commandline'
Merge branch 'sp-server-steamid'