[#42] Use same format pattern as log4j's ISO8601DateFormat

* https://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/PatternLayout.html
* https://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/helpers/ISO8601DateFormat.html
[#42] Change namespace declaration based on @jonase feedback
Merge pull request #46 from akvo/issue/42

[#42] Use same format pattern as log4j's ISO8601DateFormat