Helps troubleshooting
introduce execute_stdout

`execute_stdout` does not return the output of the process. Instead it outputs directly to current stdout (in real-time - to see what is `npm` doing now)
Make use of execute_stdout

execute_stdout makes stdout real-time
One more `verbose_mode`

`bootstrap.py` new variable `verbose_mode` to run `npm install` with --verbose option
Travis CI says long line detected

Foldede the line
Travis CI says: execute unused

Removed `execute` import
verbose_mode getter/setter
verbose_mode from config.py

Also, no return value supposed for execute_stdout
bootstrap.py also imports it
Merge pull request #901 from IgorKlopov/patch-1

Full verbosity, helps troubleshooting