Improve network layer

[irongut]

The current architecture forces the use of DownloadService which includes caching features but not all apps will require the cache.

• Create an IDownloadService interface.
• Rename the current DownloadService to CachedDownloadService.
• Use constructor injection in API services to pass the required implementation.
• Create a new DownloadService that does not implement caching. Include the connectivity check.
• Use the new DownloadService in the demo app + tests.
• Create tests for download services.

[irongut]

StandingsService is too closely coupled to the original DownloadService and the cache implementation due to the way expiry is handled.

Do not include the GetData() overload that includes dataKey & updatedKey parameters in the IDownloadService interface.

A better solution that allows the use of any IDownloadService implementation should be found.

Spun out to its own issue - #41 Decouple StandingsService from CachedDownloadService.

[irongut]

CachedDownloadService GetData() methods include expiry & ignoreCache parameters that make no sense for a non-cached download. But, the method signatures need to be the same for IDownloadService, CachedDownloadService and DownloadService.

Ideas:

• Add expiry & ignoreCache parameters to Instance method.
• Create a DownloadOptions structure / class that contains expiry, ignoreCache, cancelToken and any other relevant params. It can be passed to GetData() in all download service implementations.

[irongut]

• Add more constructors to DownloadOptions for situations where cancelToken isn't needed.