ENH: example_table, Table and parse_table available from package

Some convenience for the package level `__init__` and an example table. The example table is handy if you just want something quick to play with in an interpreter.

```python
>>> from biom import example_table
>>> print example_table
# Constructed from biom file
#OTU ID	S1	S2	S3
O1	0.0	1.0	2.0
O2	3.0	4.0	5.0
>>> from biom import Table
>>> print Table
<class 'biom.table.Table'>
>>> from biom import parse_table
>>> print parse_table
<function parse_biom_table at 0x1076d7b18>
```