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.
Python convention is to shorten numpy to np. import numpy as np and use np in the code instead of numpy