Swapped out print statements for proper logger logs

Preempting @guillermo-carrasco and @mariogiov by putting in proper logging instead of printing to stderr.
Another Python feature (?) is that while the parentheses here don't hurt, you don't specifically need them in this case and could just as soon write:

```python
bargraph_png, bargraph_pdf = plot_bars(biotype_counts, plot_basename, plot_title, True)
```

In fact, you don't even need them to make a tuple more generally -- just the commas (although of course this power should not be abused):

```python
my_string_tuple = "a", "bunch", "of", "strings"     # Still a tuple
```

Finally, to declare a _single-element_ tuple, you actually _must_ have the trailing comma:

```
>>> type( ("this is a string in a tuple!",) )
<type 'tuple'>
>>> type( ("but this is in fact just a string") )
<type 'str'>
```

![the_more_you_know](https://cloud.githubusercontent.com/assets/2773974/3525034/79d4602c-076f-11e4-943d-a6d224b65225.gif)