HM/TJ: Change x-term-drilldown requests

- Multiple values now allowed for x-term-drilldown
  ?...&x-term-drilldown=field1,field2 is equivalent to ?...&x-term-drilldown=field1&x-term-drilldown=field2
- Support for drilldown pivots
  x-term-drilldown=field1/field2 will add a pivot request.
HM/TJ: added json or extended xml format for drilldown

- backwards compatible , extendedFormat=False by default
- extended xml format supports pivots
- json format supports pivots
- x-drilldown-format parameter can set the requested format

Still WIP: new xsd.
TJ/HM: Default old 2007 drilldown format
TJ/HM: New drilldown xsd for support with facets and json format
HM/TJ: extraRequestData response with valid xsd for new drilldown-format.