Add reST documentation for open-tamil classes/methods

[arcturusannamalai]

Continue work started by @arulalant to add reST docs for open-tamil classes and methods.

Currently the following modules are exported to user as API via pip install.

1. tamil
2. transliterate
3. ngram
4. tamil.txt2ipa
5. tamil.txt2unicode
6. tamil.utils
7. solthiruthi
8. solthiruthi.stemmer
9. spell

and the following command line tools,

1. tamilwordcount.py
2. tamilwordfilter.py
3. tamilurlfilter.py
4. tamilphonetic.py
5. tamiltscii2utf8.py
6. tamilwordgrid.py

[arcturusannamalai]

As first effort : http://tamilpesu.us/apidoc/ will showcase the autogenerated sphinx docs via sphinx-apidoc command. The shell file 'build_sphinx.sh' is added via commit : 58114360659e81c151a994d9d2608c8e1813fb3c

[arcturusannamalai]

Actually, more documentation needs to be added in source files. Commit 58114360659e81c151a994d9d2608c8e1813fb3c only provides a way to showcase our docs.