Report storage fixed

- TodosReport and TodosComment must be serializable to be able to store them to build.xml.
- If two reports are concatenated and the first one has empty version, the version from the second one will be used.
Trend chart building fixed

- Collections.emptyMap() returns immutable empty map, which will generate an exception after put() method is called. Fixed by using HashMap.
- Displayed localized message updated.
- Javadoc comments updated and extended.