enable building kallisto without HDF5

[mschilli87]

Since HDF5 seems to cause several unresolved issues (#58, #79 , #82, ...?) and it doesn't seem essential for kallisto's core functionality (using the --plaintext option) it might be worthwhile to offer a build flag disabling the HDF5 related parts of kallisto allowing to build a --plaintext-only version that does not depend on HDF5 at all. This would offer a clean solution for everybody who cannot use the pre-complied binaries and doesn't need HDF5 output.

[mschilli87]

This could also help resolving (part of) #112.