Check secure feature processing enabled

[ashawley]

With respect to #177, it appears that 3 of the 8 security features in JAXP/SAX/Xerces are already enabled by default all the way back to JDK 6. They are:

• http://javax.xml.XMLConstants/feature/secure-processing is already enabled
• isXIncludeAware is already disabled
• isNamespaceAware is already disabled

These 5 are not enabled:

http://xml.org/sax/features/external-general-entities
http://apache.org/xml/features/nonvalidating/load-external-dtd
http://apache.org/xml/features/disallow-doctype-decl
http://xml.org/sax/features/external-parameter-entities
http://xml.org/sax/features/resolve-dtd-uris

[ashawley]

Tests show that the default settings enabled are not the desired ones.

[error] Test scala.xml.XMLTestJVM.issue17ExternalParameterEntities failed
[error] Test scala.xml.XMLTestJVM.issue17ExternalGeneralEntities failed
[error] Test scala.xml.XMLTestJVM.issue17DisallowDoctypeDecl failed
[error] Test scala.xml.XMLTestJVM.issue17ResolveDtdUris failed
[error] Test scala.xml.XMLTestJVM.issue17LoadExternalDtd failed