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
do not prepend 'v' to gitVersion in version validator
Merge branch 'master' of github.com:xtuple/xtuple-server into issues/373/github-tarball
fix ssh execSync -> spawnSync
Merge pull request #377 from tjwebb/issues/373/github-tarball

Issues/373/github tarball
fix xt-install chown path typo
Merge pull request #378 from tjwebb/issues/373/github-tarball

fix xt-install chown path typo
Revert "fix xt-install chown path typo"
Merge pull request #381 from xtuple/revert-378-issues/373/github-tarball

Revert "fix xt-install chown path typo"
Revert "Issues/373/github tarball"
Merge pull request #382 from xtuple/revert-377-issues/373/github-tarball

Revert "Issues/373/github tarball"
Revert "Issues/373/github tarball"
Merge pull request #383 from xtuple/revert-375-issues/373/github-tarball

Revert "Issues/373/github tarball"
Merge pull request #384 from tjwebb/issues/374/output-cease

revert to git clone; add submodule update
Merge branch 'master' of github.com:xtuple/xtuple-server into issues/374/output-cease