Add Homebrew formula for libbpg

This pull request adds a formula for `libbpg`, which is a library for encoding and decoding BPG (Better Portable Graphics). BPG is a new image format created by Fabrice Bellard, whom you may know from the FFMPEG and QEMU projects.

As noted on the [BPG web site](http://bellard.org/bpg/), the purpose of this new image format is to replace JPEG when quality or file size is an issue. Its main advantages are:

* High compression ratio. Files are much smaller than JPEG for similar quality.
* Supported by most Web browsers with a small Javascript decoder (gzipped size: 76 KB).
* Based on a subset of the HEVC open video compression standard.
* Supports the same chroma formats as JPEG (grayscale, YCbCr 4:2:0, 4:2:2, 4:4:4) to reduce the losses during the conversion. An alpha channel is supported. The RGB, YCgCo and CMYK color spaces are also supported.
* Native support of 8 to 14 bits per channel for a higher dynamic range.
* Lossless compression is supported.
* Various meta data (such as EXIF) can be included.

Need to specify the install prefix here - `system "make", "install", "prefix=#{prefix}"`. Otherwise it will install directly to `/usr/local` and not Homebrew's managed location.