Replace dynamic_cast with member functions (#210)

[inkychris]

Replaces use of dynamic_cast with member functions on schema.

This is currently a minimal implementation, since most derived classes will return nullptr, but this is easy to remedy if the general solution is acceptable. This could be left as a pure virtual function in the schema class.

Both Issue 25 and 189 tests, and the JSON schema test suite pass locally. (Windows environment so not sure how tests are supposed to be run automatically).

Library now builds successfully without RTTI.