Building shared libraries

This commit adds a Makefile to build blosc as shared library. It should allow blosc to be reused by other programs such as python-blosc or pytables, and be easier to package for Linux distributions.
(Perhaps the shared libs are useless here ?)

After linking, the shared library can be tested with:
```bash
cd tests/
# Test shared library
sed -i "s|#include \"../blosc/blosc.h\"|#include <blosc.h>|" test_common.h
export LD_LIBRARY_PATH=../
for i in *.c; do 
    gcc "$i" -lpthread -L../ -I../blosc/ -lblosc
    ./a.out
done
```