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conversion to and from scala std duration

- provides a solution for #71 based on milliseconds
- since 2.9.1 and 2.9.2 do not contain the std duration implementation,
  these would need to be dropped (is it still necessary to support
  these?)
- renames DateImplicits to ConversionImplicits
revert DateImplicits renaming
drops 2.9.1 and 2.9.2 scala version support
drops 2.9.1 and 2.9.2 scala version support (travis)
puts richSDuration in ScalaDurationImplicits
Merge pull request #72 from wookietreiber/wip-std-duration-conversion

conversion to and from scala std duration
add javadoc-badge
joda-time 2.5