Add Makefile.

* Makefile: New file.
Build libnacl.so in addition to relocatable libnacl.a.

* shared.c: New file, downloaded from
  http://oss.ucis.nl/nacl/shared/shared.c, following
  http://wiki.ucis.nl/NaCl/Shared.  I did not apply the "randombytes"
  patches.

* Makefile: Build and install libnacl.a.
update README