Only statistics are stored in build.xml

- There is a bug "Memory consumption is huge" reported in SLOCCount plugin Jira, TODOs pluging uses the same schema for storing of all parsed data to build.xml which is incorrect.
- Only small statistics that are loaded quite often and for many builds (e.g. trend image) should be stored in build.xml.
- Details should be stored in a separate file, which is loaded and parsed much less often.
- TodosReportStatistics class was created and used instead of TodosResult where appropriate.
- The plugin is now much faster and consumes much less memory if set of comments is big.