Add eslint

[bananaappletw]

Fix: #89 At first, I follow the following coding styles to fix the code.

• yield-star-spacing
• eol-last
• new-parens
• comma-spacing
• no-multiple-empty-lines
• no-cond-assign
• indent
• space-before-function-paren
• space-before-blocks
• Remove duplicate semi
• semi
• no-path-concat

Then add eslint to travis ci test.

[hemanth]