TST: Skip tests if h5py is not installed

NumPy provides a decorator that allows you to skip a test if a condition
is true.
Merge pull request #304 from ElDeveloper/skip-tests

Skip tests if h5py is not installed
Support conversion of dense to sparse
add compression to fixes#302
Merge pull request #307 from adamrp/table_factory_dense_support

Support conversion of dense to sparse
address comments from @wasade
Merge pull request #309 from teravest/addcompression

add compression to fixes#302
Add dense support for from JSON
Merge pull request #311 from adamrp/add_more_dense_input_support

Add dense support for from JSON
fix name of collapse_sample_by_metadata
for real this time
Merge pull request #312 from teravest/collapsemetadata

fix name of collapse_sample_by_metadata