Added statistic writer

This plugin generates a XML file for statistics. By now it collects counters by date of markers, deprecated elements and errors.
```                                       
 <?xml version="1.0"?>                    
 <phpdoc-stats version="2.4.0">           
   <stat date="2014-06-02T19:26:15+02:00">
     <counters>                           
       <deprecated>100</deprecated>       
       <errors>377</errors>               
       <markers>2</markers>               
     </counters>                          
   </stat>                                
 </phpdoc-stats>                          
```
This would be the base for some plots on an index page.
Related to #301