CassError is an enum

I copied the example from the README into a file example.cpp and tried to compile it.  I got this error:

    example.cpp: In function ‘int main()’:
    example.cpp:6: error: invalid conversion from ‘int’ to ‘CassError’

It's only in C++ that it's an error to initialize as `CassError rc = 0;`, so I guess that the example was intended to be plain C.  

Cleanest, though, seems to be to write code that's valid in both languages, so I've changed all examples to go `CassError rc = CASS_OK;`.