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.

> I'm happy to reach out to Fabrice via email and let him know that these two changes were required when compiling on Mac OS X.

That would be fantastic - thank you! Could you add comments above the inreplaces to mark that you've done so and you're awaiting a response?

> Regarding gsub!, just to make sure I understand the mechanics, you're saying I can replace the slashes in the first argument with quotation marks?

Yep, that's correct. The slash syntax is a regular expression literal. `gsub!` accepts either a regular expression or a string. Since you're replacing exact strings, instead of matching patterns, using quote marks is better here.