Added matrix_element_type flag for make_biom

Hey,

I hope all is well. I noticed that if there is a non-integer biom file with the matrix_element_type flag set to 'int' pieces get truncated in downstream analyses by other programs. I added a matrix_element_type flag to make_biom to allow for 'floats'. It might be best to add a test - is matrix_element_type int or float, etc?

What do you think?