Support for Tinyows WFS-T server

Hi,

I've applied some changes in order to make this plugin work with TinyOWS WFS-T server:

- With TinyOWS, the schema returned by the DescribeFeatureType request uses 'xs' as namespace, while GeoServer uses 'xsd'. For this I've added an option to define the 'xsdNs', which defaults to 'xsd:'
- the Filter created by the _whereFilter function used 'this.options.featureNS' as namespace. Perhaps that works depending on the server config, but changing this simply to 'ocg' and including that namespace works with both GeoServer and TinyOWS 
- describeFeatureUrl does not include the 'service' and 'version' parameters, which are required by TinyOWS
- I had to disable including the schema returned by 'DescribeFeatureType' to be part of xsi:schemaLocation, in the wfs:Transaction XML message.

In general I've included a few fixes (I guess some were required due to the upgrade of Leaflet itself)
- 'layer' object contains '_latlng' property, not '_latlngs'.

I've tested and verified these changes to ensure it still works with GeoServer.

Could you have a look at this, and merge this PR is you are happy with it?
There are a few things I can polish up (in further PRs) if you like. For instance the WFS version is configurable as an option, but '1.0.0'. is hardcoded in the URLs constructed for the GetFeature and DescribeFeatureType calls.