Build status badge for README

hey,

inspired by the somewhat boring work for issue #40 of manually testing buildability of V8Js against certain V8 versions I decided to configure a Jenkins instance to do the following

* continuous builds (latest versions) of V8 and V8Js
* builds of V8Js against each and every version of V8 we consider supported (currently any >= 3.21.12)

Both V8Js and V8 repositories are checked once per hour for new commits.  Besides the V8 repository is checked daily for new releases which are auto-installed and V8Js being built against.

Based on the continuous build a badge image is created and linked into the README page.

Besides this should help us notice if V8Js is once more broken by V8 API changes or we break buildability with older versions.

cheers
  stesie