Increasing Geographical behaviour precision to 8 decimal

When using the Geographical behaviour in a YML schema the latitude is stored in Mysql as a Double(18,2) instead of a Double as it was in previous versions.
With only 2 decimal places, the accuracy of the geographic point drops to about 1 km (see [Wikipedia page about Decimal Degrees](http://en.wikipedia.org/wiki/Decimal_degrees))
This patch fixes [DC-648](http://www.doctrine-project.org/jira/browse/DC-648)