allow sloppy date parsing

This patch will allow to parse dates coming from external sources that have invalid date/time syntax.

By now, such dates will fail during mapping. For example,  MySQL may create the infamous 'null' date ``0000-00-00 ...``.

	org.joda.time.IllegalFieldValueException: Cannot parse "0000-00-00": Value 0 for monthOfYear must be in the range [1,12]

In some cases, it is convenient to swallow all dates in incoming data, even with invalid dates, into Elasticsearch, for example from a MySQL river.

To enable the sloppy behaviour, the configuration of the index mapping ``index.mapping.date.sloppy`` must be set to ``true``. Default value is ``false``.