set jshintrc to warn on undefined variables
install using github tarballs instead of cloning

modified:   tasks/xt/database/package.json
modified:   tasks/xt/database/index.js
- set options.xt.gitVersion in xt.version validator function

modified:   tasks/xt/install/package.json
modified:   tasks/xt/install/index.js
- use nex-github to download and extract a tarball from github
- use updated node-latest-version module to divine node version from
  engines.node string (satisfy.sync)
- do away with centralized dist cache. download and extract directly to
  xt.userdist
pre-npm-install test xtuple module with node-latest-version
store node version in options.xt.nodeVersion
simplified build_app invocation
Merge pull request #375 from tjwebb/issues/373/github-tarball

Issues/373/github tarball