112468: consistent and defined behavior for @PmInject implementations
112468: fixing a bug in JavaDoc for @PmInject
112468: consistent and documented behaviour for @OmInject nullAllowed

fixed review results:
- allow setters without getters
- support is... for boolean getters
- cache the getter to avoid reflection for every inject
- moved some code to PrefixUtil
Review suggestions for @PmInject nullAllowed
Merge pull request #11 from pm4j/112468_PmInject_nullAllowed

112468 pm inject null allowed