Add property for number of threads to blosc compressor.

[joshmoore]

Migrate bcdev#41 for @pedson

Add a property in the blosc compressor map for the number of threads argument passed to the blosc dll. This PR will only affect write operations where the user creates their own compressor as this is a fairly straighforward change.

Usage: CompressorFactory.create("blosc", "cname", "zstd", "clevel", 1, "nthreads", 4);

A second PR will contain a further update that allows for setting the number of threads when a read operation is creating the compressor automatically. This is slightly more invasive and could be accomplished in more than one way.

[joshmoore]

Thanks, @melissalinkert. I'll get this merged and then move to the next one.