streamline getRate, use API for all currencies

[apiontek]

This builds on my previous addition of the "getApiObj" functionality Streamlined getting of rates. User doesn't need to worry about api vs web, currency variables all set the same now.

• Modified instructions to refer user to both api & web url for finding coin id.
• Modified getRate to check if passed currencyId exists in previously fetched apiObj
• Added getSingleRate, called if currencyId not in apiObj, calls api for that id.
• If by chance we still get error, calls getWebRate.
• Modified getWebRate to return only the rate, not array with rate + 'USD'

Have not implemented any error checking in getWebRate, that could be done.

[LesterCovax]

Take a look at the comment on the other PR, as this can be a lot simpler. I'd also stick to modifying the same PR if it's changing the same things.

[LesterCovax]

Fixed in 2156e552ea4314d65c61a141257285d03fba2d75