added nodeVersion support with fallback to 0.10.24

Patch for https://github.com/arunoda/meteor-up/issues/25

This implements user control of which node version from the mup.json by nodeVersion parameter.

My reasoning is as follows...

1.  Non opinionated about which version of node to install on setup.  Let user choose.
2.  Allows users to easily use this application to test their app against different versions of node.
3.  Allows ability to upgrade or fallback node versions by re-running setup changing nodeVersion param.
4.  Takes advantage of existing EJS support in nodemiral to create install-node.sh  EJS is rad.
5.  Least number of changes to code base == less disruption to end users.
6.  Fallback install of 0.10.24 if nodeVersion omitted.

Tested on Digital Ocean with 0.10.22, 0.10.23 and leaving nodeVersion out of mup.json