Refactor: Clean Up Internal dtype references

[WillAyd]

#94 fixed a pretty critical bug but exposed a flaw in internal terminology. Specifically, we create a lot of mappings back to "pandas dtypes" that we pass to the C extension, which then creates the appropriate objects

"date" however is not a real pandas dtype, but needs differentiation from datetime objects in the C extension. I hacked things together by just adding date to the "pandas_dtype" dictionary that we have and subsequently casting after creating a dataframe, but there is a cleaner way to do this