Moved type-related assertions to `pyvows.assertions.types`
Added file assertions (`assertions.types.file`)

I often find myself using files in my vows.  I thought I would cover this use case with a couple of new assertions. :)

But…there’s more than one way to check whether something is a “file”.  So how to write the assertion?

Well, one of PyVows’ strengths is the convenience of its built-in assertions.  The `to_be_like()` assertions are particularly fantastic.

In the spirit of such “fuzzy” assertions, I wrote the new file assertions with a touch of “fuzzy” logic as well.  If passed a string, the assertion checks whether the file exists.  If passed anything else, the assertion checks whether the topic is an instance of Python’s built-in file type.

I hope it’s useful!
Less redundant filenames in `tests/assertions`
Added `tests/assertions/file_vows.py`

Even with a testing syntax as awesome as PyVows, I’m not the greatest test writer. :/

But, I think this is a minimal start.  Room for future improvement includes:

- Add more test data
- In test context `WhenWeInstantiateThemAsFileObjects`, we need to make sure that open file objects are closed when that context is finished testing

On that second point, I looked into the `setup()` and `teardown()` methods.  I wanted to use `teardown()` to close the topic (an open file object)...but I couldn’t figure out how.

That’s because I have no way to pass the topic to `teardown()`.  If I want to close the topic when that context completes, `teardown()` seems like the obvious place to do it—but the topic needs to be in scope in order for that to happen.

Dammit.  ☹
Added `TODO` comment for my messy `file_vows.py`
Removed unused import
Removed unused import
File assertions accept `unicode`/`str` objects

File assertion vows also updated
Merge pull request #66 from Zearin/feature/assertion-housekeeping

Assertions: housekeeping, and new assertion (`to_be_a_file`)!