Add optional CR/LF line endings

Our bank requires that the ACH file has CR/LF instead of only LF so I added a flag to the render_to_string for using CR/LF. 

 - Add optional flag force_crlf to AchFile render_to_string.
 - Also add tests for line endings.

The tests can be run by installing nose and running nosetests.

Adding a .travis.yml would maybe make sense also to get the tests run automatically (needs some setup on travis-ci.org also)

E.g
```
language: python
python:
  - "2.7"
# command to install dependencies
install:
  - pip install .
  - pip install -r requirements.txt
# command to run tests
script: nosetests
```