pmd: fix ignored property minimumLength of rule AvoidDuplicateLiterals

The rule AvoidDuplicateLiterals ignores its property minimumLength
because it is set not until after the source code has been analyzed.
This change sets the property before the analysis.
pmd: fix #1049 Errors in "How to write a rule"
Merge branch 'master' of github.com:pmd/pmd
pmd: fix #1004 targetjdk isn't attribute of PMD task
pmd: fix typo in ReadMe.txt
pmd: AntTask support for language
- fix  #1004 targetjdk isn't attribute of PMD task
- introduce a new element, called <language>, that replaces the already existing <version/> element
Fix for bug #1056 "Error while processing" while running on xml file with DOCTYPE reference
Change the initial value of XINCLUDE_AWARE_DESCRIPTOR to false
This does validate an invalid doctype reference as in some ide generated xml files exist.
Any xml rule, which needs this property should overwrite this value with true.
pmd: verify empty if statement example
pmd: update changelog for 5.0.2/5.1.0