Extend XMLRPC API with methods: getModifiedPagesHistory, safe storePage and safe storeObject

This improvement proposes to add 3 methods to the current XMLRPC API.
These methods are the following:

/* Returns a list of all the changed pages starting from a given date, returning numberOfResults items, starting from the 'start'th element inthe result set. If fromLatest is true, more recent modifications are returned first */
public List<XWikiPageHistorySummary> getModifiedPagesHistory(Date date, Integer numberOfResults, Integer start, Boolean fromLatest)

/* These are basically equivalent to original store functions, but if checkVersion is true then a check of the current page version is done before storing the page/object. This check handles the case in which a page has been modified by somebody else after the last getPage or getObject */
public XWikiPage storePage(XWikiPage page, Boolean checkVersion)
public XWikiObject storeObject(XWikiObject object, Boolean checkVersion)