Reimplemented paragraph text objects

(1) Reimplemented paragraph text objects, now we should be more compatible with Vim for simple operations, although there are still couple of cases which doesn't work as expected:
A. extension of selection in visual mode is still not implemented (e.g. 'vipip')
B. slightly different behavior for last paragraph when file ends with EOL (e.g. deleting the last paragraph in file will leave an additional newline at the end of the file)

This code probably won't win any beauty contest, please feel free to correct me, what is wrong and what should be done differently!