Add static newInstance() method for each Cells
Set default cell size in AbstractCell
Change signature of Cell stream methods

 - Change Cell.writeCell(InputStream) to
   Cell.writeToStream(OutputStream) throws IOException.

 - Change CellParser.fromStream(InputStream) to
   CellParser.fromStream(InputStream) throws IOException.
Add CommonCellTest.java and MockCellHandler.java

 - CommonCellTest.java tests methods common to all cells.

 - MockCellHandler.java provides a way to unit test cell handling.