FIX: MB Trading dialect version 6.2.6

Add partial support for MB Trading FIX dialenct 6.2.6 as specified in:

  http://www.mbtrading.com/MBT%20FIX%20Gateway%20Specs%20(v6.2.6).pdf

Signed-off-by: Pekka Enberg <penberg@iki.fi>
FIX tools: Message encoding performance test

Add benchmark for message encoding to understand libtrading performance
better. You can run it with:

  [penberg@localhost libtrading]$ ./tools/fix/fix_perf 1000000
  encode     1000000 0.264554 µs/message

Signed-off-by: Pekka Enberg <penberg@iki.fi>
lib: Simplify buffer_sum()

Use buffer_sum_range() for buffer_sum() to simplify it.  A nice
side-effect of the simplification is increased encoding speed:

  Before:

  [penberg@localhost libtrading]$ ./tools/fix/fix_perf 10000000
  encode     10000000 0.214934 µs/message

  After:

  [penberg@localhost libtrading]$ ./tools/fix/fix_perf 10000000
  encode     10000000 0.186174 µs/message

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Use $(PYTHON) for 'fixdialectc'

Use $(PYTHON) instead of the current Python for running 'fixdialectc'.
This makes it possible to install the 'fixdialectc' dependencies into
a Python virtual environment.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Add 'requirements.txt' for Python packages

Make it easy to install Python packages with Pip:

    pip install -r requirements.txt

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Add compatibility header to 'fast_client'

'fast_client' uses 'clock_gettime' which is provided by the compatibility
layer on Darwin.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Include 'libgen.h' in 'fast_client'

'basename' is not declared on Darwin unless 'libgen.h' is included.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Include 'libgen.h' in 'fast_server'

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Merge pull request #14 from jvirtanen/fixes/darwin

Regain Darwin support (once again)
Merge pull request #15 from jvirtanen/features/virtualenv

Add support for Python virtual environments