Add responsive menu in the top navigation bar

[Kun Xi]

In phone screen, render the logo in the center and use the off-canvas menu. In desktop screen, render the logo in the left with menu aligned to the right.

[Kun Xi]

fix #15: add the responsive off-screen menu with pushy.js.

→ <<cset 1b3f3535cb7e>>

[Kun Xi]

ref #15: update the pushy menu style.

→ <<cset 273b9d99b0a1>>