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
BUG: Fix non-existant variable name

Thank you very much @ElBrogrammer
Merge branch 'master' of git://github.com/biocore/biom-format into issue-290

Conflicts:
	biom/table.py
	tests/test_table.py