Checks and installs pyusb and confirms compatibility with Latitude 7270

[kalpo]

This pull would:

• Add a check for pyusb before importing it to handle gracefully failure to load the lib
• Allows the user to install the library from the script (using pip)
• Redirects the user to the official documentation for pyusb if it fails to install it
• Adds a link to the pyusb site on pypi to the readme.md.
• Adds to the readme.md file that the script has now been tested on latitude 7270

Feel free to edit all you want. Thanks again for taking the time to make this project happen

[kalpo]

Thank you for taking the time to give such good feedbac. I'll have some time to make those little changes soon.

[jacekkow]

One thing left and I'm ready to merge it.

[kalpo]

This last commit prints to sderr for any inport error. And only specifically tells the user that pyusb is required when it is that library that we they are missing. Happy to delete that bit if you think it is a bit too much info. :)