Build process tweaks

[lmc]

Hey there, First off, thanks for your work on this, it's a pretty exciting idea to be able to use MRuby in browsers.

I did have a few issues getting started with it, especially when it was installed using gem install, which doesn't checkout the mruby submodule. I did find the current instructions a bit confusing as to which parts of the building/using/examples instructions needed to happen in the gem dir vs. my own project dir, and was not able to get it to recompile with different gems.

I also found it desirable to use a vendored version of mruby for each project, instead of having the gem provide it. This allows for different mruby-gems and build settings per-project.

I've made a few tweaks to allow using a vendored, easily-tweakable mruby install for each project, and updated the readme to reflect the instructions that got me started properly with it. Let me know how this looks, or if you'd like any changes made. Thanks again!

[blacktm]

Hi @lmc, thanks for the ideas and contribution! I just got back from some travel and checking it out now.