More cosmetic fixes for enum_services

Here's more cosmetic fixes for enum_services based on https://github.com/bbatsov/ruby-style-guide (The same guidelines Metasploit's HACKING file advices developers to follow)

Quick test:

```
meterpreter > run post/windows/gather/enum_services 

[*] Detailed output is only printed when VERBOSE is set to True. Running this module can take some time.

[+] New service credential detected: Alerter is running as 'NT AUTHORITY\LocalService'
[+] New service credential detected: AppMgmt is running as 'LocalSystem'
[+] New service credential detected: Dnscache is running as 'NT AUTHORITY\NetworkService'
[+] Loot file stored in: /Users/wchen/.msf4/loot/20140703131004_default_192.168.1.80_windows.services_870714.txt
meterpreter >
```