v8: add setVariableValue debugger command

Review URL: https://codereview.chromium.org/11415042
Review URL: https://codereview.chromium.org/11412310

This is a back-port of upstream svn commits r13122 and r13202. Should make node-inspector a little more useful in v0.10.

Suggested reviewers: @piscisaureus or @indutny