Add ability to save paired-FASTQ files

There is existing functionality for loading a pair of FASTQ files, but not for saving such files. I am in need of a tool to convert some BAM files to paired-FASTQs, so I added this capability.

I'm not an expert on the convention of having read names all end with `/1` in one file and `/2` in the other; for my purposes I want to always write them out that way and have the option of doing some strict sanity checking (all reads have a mate on the way in and out), so I added a few optional levels of verification to the loading and saving steps.

>interesting. what are SRA tools?

The [SRA toolkit](http://www.ncbi.nlm.nih.gov/books/NBK158900/) are just yet another set of tools for converting between file formats. Specifically, it converts out of the NIH SRA's file formats.

> I can see an API like you're describing making more sense, but let's flesh it out a little more first. In particular, how do you envision it being communicated on the command line?

Yeah, I don't think it's a huge deal. Perhaps let's punt on it for now (maybe open an issue) and come back to it if anyone is screaming for it later. I think I'm overthinking here.