More test cases - reached 80% coverage

[bact]

• more test cases for POS taggers, spell checkers, thai2vec, tokenizers, transliterators, and Wordnet
• handle None and empty input cases

[coveralls]

Coverage increased (+7.9%) to 80.122% when pulling af83c4d on bact:dev into 694dbf7 on PyThaiNLP:dev.

[bact]

AppVeyor failed because of network connection error- the TNC server did not respond in time.

[bact]

80% แล้ว!

Note: Slower and slower build time

As the test coverage is improving, the build time for test is dramatically slower - as test cases are now hitting with functions that use some data models - and these models (like thai2vec, thainer, etc.) have to be downloaded fresh everytime for each single build.

For discussion on how to solve this, please join #144. The proposal is, if the user chose the relevant extras_require option and if the licenses are permitted, all required data models should be installed once at the pip install time.

[wannaphong]

merge ได้เลยครับ