Fixed compilier error and warning with gcc 4.7 and enable static build.

The gcc 4.7 generates an error when compiling sodiumpp.h.The member variable overflow of class nonce will be initialized after the constructor delegation. The initialization is not necessary because the delegated constructor initializes to the same value.
Gcc 4.7 generates an warning when initializing the variable pk of class secret_key before the variable secret_bytes is initialized. The order has been changed.
I added the ability to perform an static build of sodiumpp with CMake definition "SODIUMPP_STATIC=TRUE".