Add SSL_MODE_* constants

[mhils]

Make the SSL_MODE_* constants first class citizens. (refs #190)

[coveralls]

Coverage increased (+0.01%) to 94.52% when pulling eacafe3 on mhils:add_mode_consts into 496f40d on pyca:master.

[coveralls]

Coverage increased (+0.01%) to 94.52% when pulling eacafe3 on mhils:add_mode_consts into 496f40d on pyca:master.

[coveralls]

Coverage decreased (-0.03%) to 94.49% when pulling 30b31fe on mhils:add_mode_consts into 496f40d on pyca:master.

[coveralls]

Coverage decreased (-0.03%) to 94.49% when pulling 30b31fe on mhils:add_mode_consts into 496f40d on pyca:master.

[coveralls]

Coverage decreased (-0.03%) to 94.49% when pulling 30b31fe on mhils:add_mode_consts into 496f40d on pyca:master.

[coveralls]

Coverage decreased (-0.03%) to 94.49% when pulling 30b31fe on mhils:add_mode_consts into 496f40d on pyca:master.

[exarkun]

Thanks. PARTIAL_WRITE is required to make the existing test suite pass - so the part of this change covering that constant is tested (because OpenSSL.test.test_ssl.MemoryBIOTests.test_outgoingOverflow fails if you get that value wrong).

Do you think you could add some additional tests for the other constants here, demonstrating that the desired behavior can be achieved by using them?

Thanks. Please re-open the PR when that's done.