Adding support for XML response format

[ivanprado]

Including the support for return XML results instead of JSON. The way could be adding an new parameter "format" that if it is set to XML then the results are returned in XML.

XML does not have the sense of types, so maybe it would be useful to return in a section "types" the type of each column. That would be useful for example if somebody is implementing a JDBC driver using the XML format.

Inspired by https://groups.google.com/forum/#!topic/sploutdb-users/5rTfkDi8g-I