CLDR Plural rules based plural form calculation

CLDR Plural rules based plural form calculation

* Use the plurals.xml of CLDR for the plural rules of languages
* Use plurals-mediawiki.xml to override or extend the rules inside MW
* Remove the convertPlural method in each LanguageXX.php
* Parse and load the xml files in LocalisationCache
* Use the CLDRPluralRuleEvaluator.php for parsing the cldr plural rules
  (This is taken from Translate extension and might require a replacement
   parser without using eval)
* Add getPluralRules() to make the CLDR plural rules available to JS.

PS3: More method documentation, cleanup

Change-Id: I58a9cdfe60c7b9027bf031c91370472054f04ae2