Put all public classes into a namespace to avoid pollution

[whoshuu]

All of the classes being in the global namespace is probably a no-go for a lot of applications. The right thing to do is to introduce at least one namespace out of respect for those applications.

The most obvious namespace to use is just cpr. If everything is behind that namespace, that should be enough for the most part. Tests can use the normally bad using namespace cpr to avoid onerous changes in that directory.

[whoshuu]

Labelling this issue as easy since anyone should be able to tackle this. If you're looking to contribute to this library, this would be a super easy, and impactful, place to start.