crypto:cmac calculating the Cipher-based Message Authentication Code

The ERL-82 issue requests a way to calculate a CMAC in Erlang. The
AES128 CMAC is standartized in RFC 4493 and used e.g. for message
authentication in the LoRaWAN networks.

The CMAC is implemented by OpenSSL since v1.0.1, but as @IngelaAndin
stated in response to the ERL-82, the current crypto implementation
does not include functions that call those OpenSSL cryptolib functions.

This commit introduces a new function `crypto:cmac` that calls
the corresponding OpenSSL functions and calculates the CMAC.

Only the cmac_nif is implemented. The incremental functions (init,
update, final) are not provided because the current OpenSSL does
not allow custom memory allocators like `enif_alloc_resource`.

The Erlang user guide states that at least OpenSSL 0.9.8 is required,
so I added few #ifdefs so the code is compatible with all versions.
However, the OpenSSL pages say that the pre-1.0.1 versions (0.9.8 and
1.0.0) are no longer maintained. Even the 1.0.1 will be retired by
Dec 2016. Hence I believe that adding a 1.0.1-only function like CMAC
should be OK.

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