Allow specifying binary and source tarballs on command line

We ran into a bug in node.js that was causing us some problems, so we're using a custom build of node on our production servers while we're waiting for our patch to make it into the real node.  This pull request makes it so you can specify a binary and/or source file when installing a version of node:

e.g.:

    vm install -b https://myserver/node-v0.10.12-linux-x64.tar.gz \
                   -u https://myserver/node-v0.10.12.tar.gz \
                   v0.10.12