Add __repr__ function to biom.table.Table

Function returns a string that summarizes the table's shape, class, number of
nonzero entries, and density
Improve Table's __repr__ function

Use biom.table.Table's ```get_table_density``` function, and change percent
dense representation.
Add tests for Table's __repr__ function
Merge pull request #398 from adamrp/table_repr

Add __repr__ function to biom.table.Table