print absoulte file path of caller in log

[joelurraco]

Useful when there are multiple files with the same name in the same project.

This:

2018/03/01 09:43:36 svc=unknown tp=error src=account.go:33 fn=services.(*ServerAuthService).CreateAccount msg=CreateAccount error error=password length is invalid

Becomes:

2018/03/01 09:43:36 svc=unknown tp=error src=/go/src/bitbucket.org/acbapis/microauth/services/account.go:33 fn=services.(*ServerAuthService).CreateAccount msg=CreateAccount error error=password length is invalid

[jllopis]

I am not sure if this will be generally better but understand the need. I think that adding an option in LogConfig
will do better. Use the default or just instruct the logger to output full path.

It is clear that log package needs more work...