Fixed import tamil error in pip module

I did $ pip install Open-Tamil

After that I tried to import tamil module, but it throws following error.
>>> import tamil
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/Open_Tamil-0.
2.8-py2.7.egg/tamil/__init__.py", line 9, in <module>
    from . import txt2unicode
ImportError: cannot import name txt2unicode

So to corrected it by made changes in __init__.py import statement of tamil module.

I request you to merge this and re-release Open-Tamil in pip, along with issue#5 .

Updated 2 lines in credits section of tamil/txt2unicode/README file.