Package generation for Ubuntu and CentOS

Summary:
I put together a script to assist in the generation of deb's and
rpm's. I've tested that this works on ubuntu via vagrant. I've included the
Vagrantfile here, but I can remove it if it's not useful. The package.sh
script should work on any ubuntu or centos machine, I just added a bit of
logic in there to allow a base Ubuntu or Centos machine to be able to build
RocksDB from scratch.

Example output on Ubuntu 14.04:

```
root@vagrant-ubuntu-trusty-64:/vagrant# ./tools/package.sh
[+] g++-4.7 is already installed. skipping.
[+] libgflags-dev is already installed. skipping.
[+] ruby-all-dev is already installed. skipping.
[+] fpm is already installed. skipping.
Created package {:path=>"rocksdb_3.5_amd64.deb"}
root@vagrant-ubuntu-trusty-64:/vagrant# dpkg --info rocksdb_3.5_amd64.deb
 new debian package, version 2.0.
 size 17392022 bytes: control archive=1518 bytes.
     275 bytes,    11 lines      control
    2911 bytes,    38 lines      md5sums
 Package: rocksdb
 Version: 3.5
 License: BSD
 Vendor: Facebook
 Architecture: amd64
 Maintainer: rocksdb@fb.com
 Installed-Size: 83358
 Section: default
 Priority: extra
 Homepage: http://rocksdb.org/
 Description: RocksDB is an embeddable persistent key-value store for fast storage.
 ```

 Example output on CentOS 6.5:

 ```
 [root@localhost vagrant]# rpm -qip rocksdb-3.5-1.x86_64.rpm
 Name        : rocksdb                      Relocations: /usr
 Version     : 3.5                               Vendor: Facebook
 Release     : 1                             Build Date: Mon 29 Sep 2014 01:26:11 AM UTC
 Install Date: (not installed)               Build Host: localhost
 Group       : default                       Source RPM: rocksdb-3.5-1.src.rpm
 Size        : 96231106                         License: BSD
 Signature   : (none)
 Packager    : rocksdb@fb.com
 URL         : http://rocksdb.org/
 Summary     : RocksDB is an embeddable persistent key-value store for fast storage.
 Description :
 RocksDB is an embeddable persistent key-value store for fast storage.
 ```

Test Plan:
How this gets used is really up to the RocksDB core team. If you
want to actually get this into mainline, you might have to change `make
install` such that it install the RocksDB shared object file as well, which
would require you to link against gflags (maybe?) and that would require some
potential modifications to the script here (basically add a depends on that
package).

Currently, this will install the headers and a pre-compiled statically linked
object file. If that's what you want out of life, than this requires no
modifications.

Reviewers: ljin, yhchiang, igor

Reviewed By: igor

Differential Revision: https://reviews.facebook.net/D24141