Add Cython-accelerated row-batch decoding

[TheDistributor]

Add Cython-accelerated row-batch decoding

Compiles the server result-batch decode loop (pynuodb/_fetch.pyx) to
C via Cython, replacing the pure Python per-row/per-column dispatch
in EncodedSession.fetch_result_set_next(). If the cython library is not
available at run time or if compiler tools are not available at package
build time, it gracefully falls back to the python native version.

Many tests added to test that it produces byte-identical output to the
native python version, and for many pathological cases.

Benchmark results compared to master:

========================= 9 passed in 79.98s (0:01:19) =========================
Test                                      master min (ms)  branch min (ms)     delta (ms)    delta %
----------------------------------------------------------------------------------------------------
test_fetchall_binary_types                         10.256            3.073         -7.183    -70.04%
test_fetchall_large                                77.005            6.123        -70.882    -92.05%
test_fetchall_mixed_types                         416.255          125.674       -290.581    -69.81%
test_fetchall_small                                 3.884            0.371         -3.513    -90.45%
test_fetchall_wide                                 47.262            2.129        -45.133    -95.50%
test_fetchmany_large                               76.295            6.306        -69.989    -91.73%
test_fetchone_loop_large                           84.153           11.685        -72.468    -86.11%
test_insert_large                                 184.796          185.560         +0.763     +0.41%
test_insert_small                                  12.428           12.479         +0.051     +0.41%

Insert related improvements will come in a subsequent commit.