Squashed 'src/univalue/' changes from daf1285..16a1f7f

16a1f7f Merge #3: Pull upstream
3f03bfd Merge pull request #27 from laanwj/2016_09_const_refs
5668ca3 Return const references from getKeys, getValues, get_str
cedda14 Merge pull request #28 from MarcoFalke/patch-1
9f0b997 [travis] Work around osx libtool issue

git-subtree-dir: src/univalue
git-subtree-split: 16a1f7f6e9ed932bec7c7855003affea1e165fb5