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
remove scaladocBranch Key. use "git rev-parse HEAD"
add DurationSpec
strip test scope libraries from pom.xml
Added static method wrappers
Merge pull request #74 from tkawachi/static

Added static method wrappers
Scala 2.11.3
Revert "Scala 2.11.3"

https://groups.google.com/d/msg/scala-internals/SSD9BNJaFbU/rACBkHrs2JEJ

This reverts commit 52da8966250fab6b331f15529a0ec84fd356ea0b.
Scala 2.11.4
nscala-time 1.6.0
Merge pull request #75 from kmizu/release/1.6.0

nscala-time 1.6.0
About dropping Scala 2.9.1 and Scala 2.9.2 support
Fix documentation