Support Avro INT, LONG and DOUBLE for all XML numeric primitive types

Many XML types could be better represented as Avro INT, LONG or DOUBLE
instead of String.

The only XML numeric type that cannot be represented as Avro LONG is 
the unsignedLong: this is due to the lack of “unsigned types” in
Avro and thus the maximum unsignedLong value would overflow Avro LONG.
Merge branch 'avro-int-type' of https://github.com/lucamilanesio/xml-avro into lucamilanesio-avro-int-type
Merge branch 'lucamilanesio-avro-int-type'