Petpy

- Fix to the `shelter_get()` method for handling empty responses when no shelters returned for when
the parameter `return_df = True`.
- Fix to getting pets available at a shelter with `shelter_get_pets()` when the parameter
`return_df = True` when the given shelter does not return any available animals.

- Refactoring of the library to clean up code files.
- Fixed a bug with the `shelter_get_pets()` method that caused an error to be thrown when there is only
one pet record and the parameter `return_df = True`.
- Many changes to simplify expressions and internal code within methods.
- The `Petfinder` class method names and parameters have been renamed to be PEP8 compatible. I apologize as this will
cause backward compatibility issues upon updating for anyone using previous versions. The original intention of the
naming was to reflect the Petfinder API's method names as closely as possible, but after further consideration and
given the relatively short life of the library, I think the change is necessary for the future growth and maturity
of the package.
- How the methods interact with the Petfinder API is unchanged. Thus there is no immediate need to update your
version of petpy if it will break any current code.
- The Github repo README has been updated with the new API methods.
- Below is a table detailing the changed method names:

| Previous Method Name | New Method Name |
| -------------------------- | ---------------------- |
| pet_getRandom() | pet_get_random() |
| shelter_getPets() | shelter_get_pets() |
| shelter_listByBreed() | shelter_list_by_breed() |

- The following lists the method parameter names that have changed with the release:

| Previous Parameter Name | New Parameter Name |
| ---------------------------- | ------------------------- |
| petId | pet_id |
| shelterId | shelter_id |

- This release removes pandas as an installation requirement for the package. Although pandas is
required to convert the API results into a DataFrame, this is optional and not necessary to the
building or use of the package itself.

- Calls that return JSON results when using the `pet_find()` method when `return_df=True` are now
adequately handled and an empty pandas DataFrame is returned. This result can happen when searching for a
particular breed of animal that is currently not available in the Petfinder database.

- The paged results should now cap at Petfinder's 2,000 search limit consistently.
- The methods `shelter_get()` and `shelters_get()` now handle shelters that have opted-out of having
their information available in the Petfinder API.

- Paged results will now reach Petfinder's 2,000 records per search limit. Before, if the next paged result would
equal or exceed 2,000 results the call would end, and the results would be returned. For example, if the parameters
`pages` is 10 and `count` is 200, 2,000 records will now be returned, whereas previously 1,800 would be returned.