Added support for compressed source packages

Added a new flag called keepSourcesCompressed which changes the behavior of the symbols and sources provider. When set to true, symbol packages are stored as the original (compressed) nupkg file, saving of space and lowering the file count.

Any requested source file is uncompressed and served on the fly from the nupkg file.

I got the idea from the NuPeek repository project, where this behavior is the default.

Hint: If you set keepSourcesCompressed to true on an existing repository, the server won't be able to find any previously pushed sources. A small tweak in the code could solve this by providing a fallback mechanism, trying to find the uncompressed sources if the nupkg file can't be found