WebSearch: partial SRU protocol support

* Implements partial support to SRU protocol as described in
  <http://www.loc.gov/standards/sru/index.html>.
  (addresses #1153)

* Operations Search/Retrieve and Explain are supported but operation
  Scan and CQL language are not supported.

* Example URL: /sru?operation=searchRetrieve&version=1.2&query=Model

Signed-off-by: Vangelis Banos <vbanos@gmail.com>
WebSearch: SRU encoded for XML

* Properly encodes for XML strings embedded in SRU responses.
  (PR #2311)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>