DenseNet feature pyramid computation

## DenseNet PR current state / TODOs (for discussion)

### explanatory tech report:
[DenseNet: Implementing Efficient ConvNet Descriptor Pyramids](http://arxiv.org/abs/1404.1869)
Forrest Iandola, Matt Moskewicz, Sergey Karayev, Ross Girshick, Trevor Darrell, and Kurt Keutzer

### additional integration notes (see main notes below inlined from the DENSENET_MERGE_TODO file)
- last rebase was a while ago now; there were some delays getting our tech report polished up
- in the mean time, some work on replacements for the GPL'd code is in progress; see:
 - BLF: https://github.com/moskewcz/boda/blob/master/src/blf_pack.cc
 - CNN padding/support/sizes calcs: https://github.com/moskewcz/boda/blob/master/src/conv_util.cc
 - hosting repo / framework: https://github.com/moskewcz/boda/

### inlined from: DENSENET_MERGE_TODO
list of the issues blocking the merge of the DenseNet feature branch:

- critical
 - replacement of GPL'd code (including removal from history)
 - update build process / Makefile to match current practice 
 - tests
 - trivial: remove DenseNet README.md header
 - trivial: remove this todo file

- unclear neccessity, semantics, and/or priority
 - general cleanup of commit sequence (probably mostly squashing)
 - input interface changes (i.e. jpeg filname as input -> ?)
 - output interface changes (?, but probably something: image support size, multiple layer output, alignment, etc.)
 - if still reading image files after any iface changes and removal of GPL code, use XX instead of YY