Fact for Express Service Code
Service Tag seems to have 5 or 7 characters

According to: http://linux.dell.com/libsmbios/main/SystemInfo_8h.html
"const char * 	SMBIOSGetServiceTag ()
 	Returns the 5 or 7 character Dell Service Tag."
rename fact to dell_expressservicecode
use confine
easier to read fact name
Merge pull request #33 from mremy/add_dell_express_service_code

Fact for Express Service Code