Allow usage of AlchemyAPI

[tmaiaroto]

Alchemy is a really convenient API that provides sentiment analysis, entity extraction, keyword extraction and a few other things. It's a great compliment to Social Harvest, but it requires a subscription. So it won't be for everyone (depending on budget and volume of harvest data). So it should be an optional service.

[tmaiaroto]

The direction of Social Harvest is heading toward not relying upon any external APIs. For example geocoding has been moved to an internal package. Sentiment analysis and gender detection are also both internal packages that use in memory training data.

A future version of Social Harvest may allow for 3rd party integrations, but this current initial release will focus on keeping everything internal to the package.