Add support for linked XSD files from WSDL

[InfoSec812]

Currently, when I try to load the WSDL for VMware's vSphere SOAP API, I get the following error:

** exception error: no match of right hand side value {error,"Include file not found query-messagetypes.xsd"}
     in function  scrub:addSchemas/4 (src/scrub.erl, line 495)
     in call from scrub:parseWsdls/8 (src/scrub.erl, line 417)
     in call from scrub:initModel2/8 (src/scrub.erl, line 376)

It appears to stem from the fact that the WSDL references some external XSD files for validation and scrub/erlsom is unable to go and fetch them to complete the validation of the WSDL.