Delay importing requests so we don't get `ImportError` when running setup.

`setup.py` needs to access `wepay.VERSION`, and the `wepay` module imports
`wepay.api` which depends on `requests`, but at the time setup is
running, `requests` library has not been installed (will be installed as
part of `install_requires` specified in setup.py). This fix breaks the
circular import by delaying importing `requests` until a `WePay` object
is instantiated.
Merge pull request #14 from kevinjqiu/fix-setup

Delay importing requests so we don't get `ImportError` when running setup