Preliminary (not yet working) support for REST Activities
Fix overly strict interpretation of XML

http://www.w3.org/TR/REC-xml-names/ only recommends that the prefix "xml"
not be used for any element. It does state that the prefix "xmlns" MUST
NOT be used.
Add working HTTP (REST) and XPath activities

Reimplemented input and output mapping, to allow activity to identify which
ports are mapped. This allows the REST Activity to detect whether any of
the optional outputs are used, and enable them.
R Activity does not store any task context

The R Activity would be modified by use of implicit RExpression variables.
If the R Activity was shared, extra variables would show up in all instances.
This commit fixes that to keep the R activity free of any connection info