implement Blosc compression for #173
no need to check for HDF5 1.8 in blosc.jl, since that is required by HDF5.jl anyway
add heuristic auto-chunking if chunk is not specified for blosc; add compress=true option to JLD
Pkg.add Blosc in travis
added tests for Blosc-compressed JLD
address comments by @timholy
add documentation of new compression options; change `"compress"` default to `"blosc"` in the HDF5 interface to correspond to `compress=true` in JLD
Merge pull request #174 from stevengj/blosc

implement Blosc compression