fixed request generation for different cases

refs #28
explicit point to override urlopen()

refs #24
python3 doesn't have Request.add_data() method

so we use .data property directly. seems to work everywhere

refs #24
base64 works only on strings, so encode()

refs #24
revert next/__next__ change introduced by @wikier

it looks like it is not needed with 2to3 and just leads to recursion on py3

refs #24
Merge pull request #30 from indeyets/py3tests

Fixes for Python3 tests. refs #24