Mock data providers

@ABW @bwrona Please take a look:

• introduced two new classes: MockBlockProvider and MockVopsProvider
• possibility to inject artificial data to block and vops using data from mock providers
• data for mock providers is stored in json files, examples provided
• added command line options to set paths for mock json files
• provided path can also be a path to a directory. Direcotry will be scanned for json files and all json files from given directory will be loaded

I didn't want to mess with block streamer to much so:

• blocks in json file must be in sequence, starting with block next to last one synced from hived,
• json file must have three more blocks than required to fill the queue (just like in example file)