improve(Tokenizer) : add LexerTokenizer (refs #2)

Refactor package structure => remove kr.ac.snu
Set default Tokenizer as LexerTokenizer, move other Tokenizers to test dir
Tokenization time was reduced about 23%.
In case of sample docs,
  - by spaces : 5.7s
  - by lexer  : 6.1s
  - by regex  : 7.9s.