switched to having the icon templates use a shared function

also now the switch between iconReferenceStyle == "inlineLiteralXml" and iconReferenceStyle == "dataUri" is now working
  * data urs with base64 encoded svg data are now usable
  * inlineLiteralXml is the default

change also allows the stylesheet parameter to be passed in to the main stylesheet (stix-to-html.xsl) and used in the (now imported) icons.xsl

this supports issue #23
now supporting iconReferenceStyle == "relativeUri"

also added parameter iconExternImageBaseUri that allows user to specify a relative or absolute uri to where the svg logo images are stored.

supports issue #23