Added txt2unicode, txt2ipa modules

Dear [Muthiah Annamalai](@arcturusannamalai),

I merged my two projects [txt2unicode](https://github.com/arulalant/txt2unicode) and [txt2ipa](https://github.com/arulalant/txt2ipa) in your open-tamil project under 'tamil' module.
Also I added examples of txt2unicode and txt2ipa under examples directory.
Unittest for txt2unicode script is added in test module.
Added sections about txt2unicode and txt2ipa in mail README.md file of open-tamil.
Right now txt2unicode supports with 20 Tamil encodes for encode2unicode and vice versa with auto find encode functions.

PS : I didn't delete any of existing code line.

Thanks,
Arulalan.T