Generate a shared library so that user programs can use it on Linux

The static library, libkytea.a, has been created since we had switched.
The purpose is to create the executables: kytea and train-kytea, and to be able
to use the C/C++ APIs from the users.  It turns out on Linux, the library is
a "thin" archive file.  This is because gyp emits the Makefiles in which
GNU ar runs with -T option on Linux (See https://codereview.chromium.org/2841004
for the details), which is not controllable by gyp.  Then, the reasonable
solution would be to create a shared (or dynamic) library on all platforms
instead of providing static and shared libraries depending on the platforms.