DateField timezone handling

The Solr DateField appears to only be partially ISO 8601 compliant.

The DateMathParser requires Timezone modifications to be in the format "+nMINUTES", "+xHOURS", "+yDAYS" etc.

http://lucene.apache.org/solr/api/org/apache/solr/schema/DateField.html

ISO 6801 states that timezone modifications should be in the format +00:01, +01:00

http://en.wikipedia.org/wiki/ISO_8601#Time_offsets_from_UTC

It would be useful if Solr DateField could parse both (I presume there's a reason for +nMINUTE etc somewhere in Java.)