make_optparse now includes options for setting header information

These changes introduce a new Command, ```CodeHeaderGenerator```, which generates a list of header lines, e.g.:

```
#!/usr/bin/env python
from __future__ import division

__author__ = "Greg Caporaso"
__copyright__ = "Copyright 2013, Greg Caporaso"
__credits__ = ["Greg Caporaso"]
__license__ = "BSD"
__version__ = "0.0.1"
__maintainer__ = "Greg Caporaso"
__email__ = "gregcaporaso@gmail.com"
```

This new Command does not have an optparse interface associated with it. ```MakeCommand``` and ```MakeOptparse``` inherit from ```CodeHeaderGenerator```.

```MakeCommand``` and ```MakeOptparse``` now return lines of output instead of a single string containing the formatted results. ```credits``` is now a list instead of a comma-separated string.

Fixes #151.