Some improvements and fixed issues

[GastonZalba]

This PR has:

• New prop loader: added a customizable loading indicator (by default it's a css spinner) #12
• Added the ability to update tokens before they expire using an interval #14
• New component GoogleDataRT: added RealTime support with customizable output using prop customOutput #3
• Workaround to display the auth button after is unmounted (the api renders it only once per page load) #15
• Charts are redrawn when the window is resized #9
• Added prop userInfoLabel to customize the auth button message displayed before the logged in user's email address
• New prop errors: the errors returned by the api can be displayed inside the widgets (insufficient quota, bad view id, missing/wrong parameters, etc)
• New css file (optional) with some basic css style to allow some minimal interface states (for loading indicator, showing errors, etc)
• Replaced legacy method componentWillUpdate
• Added a default chart configuration if nothing is provided
• Updated packages with npm audit fix: 1812 vulnerabilities (732 low, 75 moderate, 1002 high, 3 critical). 1 low remains unfixed because is a potentially breaking change with gh-pages
• Updated demo/src/index.js and added an extra demo with a token configuration (npm run-script start:token-demo)
• Updated readme and example image with the improvements

The idea was to change only a few things, and they ended up being quite a few. So, let me know if maybe is better break this in multiple PRs. Hope it's usseful.