Replaced buildJaxbContextString() with Iterators.join()
Added null check when loading the XSD schema
Refactored TaxiiXml and TaxiiXmlFactory to reduce memory usage

Now only TaxiiXmlFactory keeps a list of packages; TaxiiXml only
keeps a reference to the created JAXBContext.
Added another constructor that takes a list of packages.
Moved list construction and appending to newJaxbContext(),
renamed from initializeJaxbContext().
Removed TaxiiXml.newInstance(), as it wasn't adding any value.
Resulting TaxiiXml design is more stateless and uses less memory.
Method signatures use List now instead of ArrayList.
Generated stylesheet from schematron for additional validation

Modified TaxiiXml to use stylesheet for validation instead of custom
code.
Enhanced ValidationErrorHandler to detect how stylesheet triggers
errors.
Moved and renamed a test file used by StatusMessageValidationTest.
Constrained Schematron to XSLT2 Basic Profile

Fixed TaxiiXml.checkConformance() so that it works.
All unit tests pass with this commit.