Prevent crashes when working with ES2015 symbols

[codeheroics]

redux-devtools-inspector crashed when using symbols.

The two following things caused the crashes:

• javascript-stringify did not support symbols whereas the previously used jsan did. Here's the PR I sent them to fix this (merged).
• getShortTypeString in ActionPreview.jsx did not support Symbols, so it tried using them as strings, which is not supported and caused the error Uncaught TypeError: Cannot convert a Symbol value to a string

This corrected, there is still a problem as Symbol values are not shown in JSONTree. Here's the PR I sent for this

I think this is what causes #17, though I'm not sure it is the only cause. I got it while sending a React element in an action payload, and React elements have a $$typeof: Symbol.for('react.element') property

[alexkuz]

Thank you so much!
jsan always generates valid JSON (which we don't need here, actually), so it won't show functions, unless we use the special jsan format. I suggest we wait for approval of that PR in javascript-stringify.

[codeheroics]

A new version of javascript-stringify with my modification was released, so I just updated the PR!

[alexkuz]

@codeheroics published in v0.3.3. Thanks again!