Added tests and implementation for `Duration.multipliedBy(long factor)`
* added tests and implementation for `Duration.dividedBy(long divisor)`
* added tests and implementation for `FieldUtils.safeDivide`
* renamed multiplication tests to match method name
* renamed parameter name of `Duration.multipliedBy` to match Java 8 API
added tests and implementation of `Duration.negated()`
added documentation for all newly introduced methods
Merge branch 'master' of https://github.com/bjoernpollex/joda-time into bjoernpollex-master
Add Duration multipliedBy/dividedBy/negated

Tweak pull request
Fixes #72