Address @khandelwal's comments.

  - Switched from strndup to StringValueCStr.
  - Added comments about expected arguments for vararg functions.
  - Cleaned up some comments.
  - Updated sign_with_key to correctly specify c14n transform.
  - Updated unittest golden files to reflect c14n transform being
    specified in the signature transforms.
Mark system certificate test pending.

This test is impossible to execute without admin intervention.
Marking it pending to require manual intervention.
Add ignores for vim swap files.
Tighten up XSLT and external reference security.

  * Only allow empty or local document URI in signature reference.
    This prevents libxml2 from reading random bits of the filesystem
    based on a URI.
  * Disable all XSLT features that use filesystem or network access.
  * Redirect xmlsec1 logging away from stdout. Embedd errors into
    the exception message.
  * Add tests that assert the removal of filesystem and network access.
  * Rename key manager facetory methods to be more descriptive.
  * Add comments.

Still missing tests for possible XSLT abuse.