Makefile: Shared library support

Fixes #29.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Ignore 'libtrading.dylib'

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Ignore 'libtrading.so'

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Merge pull request #30 from jvirtanen/fixes/gitignore

Update '.gitignore'