Create python script to examine leveldb datum

Create `examine_leveldb.py` to dump datum contents of a leveldb database. The `datum` protobuffer type is defined in `$CAFFE_ROOT/src/caffe/proto/caffe.proto` ([link](https://github.com/CellScope/caffe/blob/master/src/caffe/proto/caffe.proto#L20)).

Example usage:

* Ensure your $CAFFE_ROOT environment variable is defined to point to the root caffe git directory on your system, e.g., in `~/.bashrc`

```bash
export CAFFE_ROOT=/path/to/caffe
```

* First create the CIFAR leveldb database:

```bash
cd $CAFFE_ROOT/data/cifar10
./get_cifar10.sh
cd $CAFFE_ROOT/examples/cifar10
./create_cifar10.sh
```

* Then run with default arguments:

```bash
python $CAFFE_ROOT/python/examine_leveldb.py
```

* Get help the usual Python way

```bash
python $CAFFE_ROOT/python/examine_leveldb.py --help
```

Example output:

```
Key: 00000
channels: 3
height: 32
width: 32
label: 6
data length: 3072
float_data length: 0
```
![figure_1](https://cloud.githubusercontent.com/assets/1332812/4168914/1a2f00ec-3522-11e4-9fd8-3dc5ae72c94d.jpg)

*TODO:* Convert to a Python class instead of this procedural junk. I may or may not ever get around to doing that.