Add a mechanism to allow the emscripten virtual heap size to be specified at run-time by applications rather than having to change it at compile-time. Improve the documentation especially the main README

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/branches/fadams-javascript-binding@1626584 13f79535-47bb-0310-9956-ffa450edef68