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