Add make install

Summary:
Add make install.  If INSTALL_PATH is not set, then rocksdb will be
installed under "/usr/local" directory (/usr/local/include for headers
and /usr/local/lib for library file(s).)

Test Plan:
Develop a simple rocksdb app, called test.cc, and do the followings.

make clean
make static_lib -j32
sudo make install
g++ -std=c++11 test.cc -lrocksdb -lbz2 -lz -o test
./test

sudo make uninstall
make clean
make shared_lib -j32
sudo make install
g++ -std=c++11 test.cc -lrocksdb -lbz2 -lz -o test
./test

make INSTALL_PATH=/tmp/path install
make INSTALL_PATH=/tmp/path uninstall
and make sure things are installed / uninstalled in the specified path.

Reviewers: ljin, sdong, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23211