Fixed number of rows

Populate current number of rows from data source
Fix retain cycle on block.

cell.onTextEntered block is capturing self
Fixed modify number of items in data source while editing

When it uses insertObject:atIndex method from NSMutableArray and the
index is already occupied, the objects at index and beyond are shifted
by adding 1 to their indices to make room. Therefore when edit an row
secont time self data add new item.
Merge pull request #9 from martinezdelariva/master

Several improvements on chapter 11