Add JsonParser feature to ignore a trailing comma (fixes #118, #323)

[bdhess]

Design discussion in #323

Diff of the before/after benchmark results:
bdhess/jackson-benchmark-logs@590fa18?diff=split

[cowtowncoder]

@bdhess I think this is pretty good; I may make some minor changes but nothing major. Thank you for doing benchmarks too; from quick glance numbers looked quite similar. I can also run tests locally myself to double-check, but from changes I think we should be good.

So: I'd be happy to merge this PR to be included in master for 2.9.

One last thing however: before merging the first contribution, we need CLA (unless I already asked and got one; I tried to verify). It's here:

https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf

and the usual way is to print, fill & sign, scan/take photo, email to info at fasterxml dot com.
Once we get this, I'll go ahead and merge this.

Looking forward to merging!