Update lib/OpenLayers/Format/SOSGetObservation.js

Adding gml:TimePeriod support for eventTime parameter (writer only), for example:
```javascript
eventTime: {
	timePeriod:{
		beginPosition:"2012-12-01T01:01:00+00",
		endPosition:"2013-01-10T23:30:30+00"
	}
}
```
Will produce:
```xml
<eventTime>
	<ogc:TM_During xmlns:ogc="http://www.opengis.net/ogc">
		<ogc:PropertyName>urn:ogc:data:time:iso8601</ogc:PropertyName>
		<gml:TimePeriod xmlns:gml="http://www.opengis.net/gml">
			<gml:beginPosition>2012-12-01T01:01:00+00</gml:beginPosition>
			<gml:endPosition>2013-01-10T23:30:30+00</gml:endPosition>
		</gml:TimePeriod>
	</ogc:TM_During>
</eventTime>
```