Use JSStrings and macros from JSExpr

[rdeits]

Updated version of #89

This is a step towards JuliaGizmos/WebIO.jl#48 . I didn't introduce the trait yet, but it will be trivial to do once Blink depends on WebIO.

@shashi I made some guesses about the right way to do this. The tests pass, and the p5.js example from WebIO works, and MeshCat.jl works, but I'm still guessing I got at least something wrong here 😉

[shashi]

Appveyor says

   Evaluated: Any[140, 200] == [100, 200]

I get this test failure on my Fedora machine as well

  Expression: size(w) == [150, 100]
   Evaluated: Any[150, 101] == [150, 100]

[pfitzseb]

Looks good to me.
The appveyor failure is unrelated (I'll look into that when I find the time). Tests pass locally on Linux -- I suspect the off-by-one error is some weird pixel measuring (and rounding) thing.

Do you think we should merge this now or do you want to keep working on this branch, @rdeits?

[rdeits]

I don't have any more changes planned. My only concern is that I wanted to make sure that my assumption that what was previously Blink.jsstring(x) is now more like string(JSExpr.jsstring(x)...) is correct. That is, jsstring now returns a collection which needs to be concatenated to make a single JS string. is that assumption right?

[rdeits]

bump?