Closes #1233

- Handle path-separators properly. Use "path.sep" instead of "/".
  Or use "require.resolve()" if possible
- Use "execFile" instead of "exec" to run the Handlebars executable.
  This prevents problems due to (missing) shell escaping.
- Use explicit call to "node" in order to run the executable on Windows.
- Add "appveyor"-CI in order to run regular tests on Windows.

closes #1391

uglify-js is an optional dependency and should be treated as such.
This commit gracefully handles MODULE_NOT_FOUND errors while loading
uglify.

- Check for existing uglify-js (and load uglify-js) only if minification
  was activated
- Use "require.resolve" to check if uglify exists. Otherwise, a missing
  dependency of uglify-js would cause the same behavior as missing
  uglify-js. (Only a warning, no error)
- The code to load and run uglify is put into a single for readability
  purposes
- Tests use a mockup Module._resolveFilename to simulate the missing module.
  This function is used by both "require" and "require.resolve", so both
  are mocked equally.

(cherry picked from commit d5caa56)

Welcome to 2017!

(cherry picked from commit 33773c2)

closes #1376

(cherry picked from commit ce3cd8a)