Add caching support of requests

[stijnvanhoey]

Add the integration with the requests-cache package to provide caching on the request level, see #24. Hence, identical requests are retrieved from cache.

Some assumptions were made to have a first version:

• use the default sqlite file storage of requests-cache as back end for the cache in the 'current' location
• default of 7 days storage of queries
• clear_cache function to provide a way to clear the cache
• cache on the level of a request (not the resulting datafram). Hence no 'intelligence' with respect to overlapping time series of the same ts_id or requests of the same ts_id with different returnfields.

@Sachagobeyn and @nielsdevleeschouwer can you have a check if this suffice for your usage?

[nielsdevleeschouwer]

Useful addition, seems to work fine.