baseline changes to help (JRuby) development

[kares]

these were originally part of #361

the most relevant change is that we weren't able to run tasks test_pure and test_ext as intended.
... one needed to do: env JSON=pure bundle exec rake test_pure

[kares]

// cc @headius please review and ping @hsbt for merging

[kares]

some progress on master from @flori: 0e97fd4 ... wondering if we can get the remaining 2 commits in?

[kares]

... could we get this in that here, for future json work or do we have to do smt differently?

[olleolleolle]

These changes make JRuby builds possible and more practical! LGTM!

[headius]

Sorry this flew under my radar. The changes look fine.