ssl: add aes gcm cipher

This adds support for the AES GCM cipher suites. AES-GCM is much faster than normal AES+MAC. On Intel CPU's with AES-NI it is comparable to RC4 in terms of throughput.

The cipher support in crypto is based on OpenSSL's EVP contexts. The EVP context where chosen over the more generic CRYPTO_gcm128_xxx functions for performance. The CRYPTO_gcm128_xxx functions use a generic implementation while the EVP context can use hardware optimized variants (AES-NI or padlock instructions) on supported CPUs'

Some benchmarks on an Intel(R) Core(TM) i7-2760QM CPU @ 2.40GHz:
```
The 'numbers' are in 1000s of bytes per second processed.
type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes

openssl speed aes-128-cbc (generic functions):
aes-128 cbc     105336.34k   112751.42k   114742.10k   115345.41k   113988.95k

openssl speed -evp aes-128-cbc (EVP with AES-NI):
aes-128-cbc     615445.91k   660332.93k   670884.44k   665051.48k   669777.92k

openssl speed -evp rc4:
rc4             348612.51k   609902.38k   748361.73k   782705.19k   788460.89k

openssl speed -evp aes-128-gcm (EVP with AES-NI):
aes-128-gcm     310758.55k   785113.43k  1156720.81k  1277712.38k  1323431.25k
```