Easily convert the binary protocol buffer mean file to a Matlab matrix!

I've seen many people confused and wondering if there's a straightforward way to convert these binary mean files to a Matlab matrix. So I simply added a new function to the Caffe Matlab wrapper that does exactly that.

It's fairly simple to use:
```matlab
caffe('read_mean', 'path to the mean file');
```
The output is a matrix.

```
>> mean_mat = caffe('read_mean', 'caffe/examples/cifar10/mean.binaryproto');
Warning: Remember that Caffe saves in [width, height, channels] format and channels are also
BGR! 
>> whos mean_mat
  Name           Size              Bytes  Class     Attributes

  mean_mat      32x32x3            12288  single              
 
```