Support for new compressors in Blosc 1.3

Hi Antonio.  I have just seen that you created a first release branch.  Here you have a PR for including the full Blosc 1.3 sources in PyTables.  With this, you can be sure that the additional compressors can be used in PyTables.

The strategy is that a Blosc library is first looked up in the system, and if it is not found, the internal Blosc sources are compiled and integrated with PyTables extensions.

The way to specify the additional compressors is easy: "blosc:lz4" will select the LZ4 compressor and 'blosc' and 'blosc:blosclz' are equivalents.