More documentation for IO

fixes #630 
It'd be great to have a high-level intro to grab the reader's attention. Maybe something like:

Reading and writing files (I/O) can be a complicated task:

* A file format can sometimes be read into more than one in-memory representation (i.e., object). For example, a FASTQ file can be read into an `skbio.SequenceCollection` or `skbio.Alignment` depending on the file's contents and what operations you'd like to perform on your data.

* A single object might be writeable to more than one file format. For example, an `skbio.Alignment` object could be written to FASTA, FASTQ, QSEQ, PHYLIP, or Stockholm formats, just to name a few.

* You might not know the exact file format of your file, but you want to read it into an appropriate object.

* You want to read multiple files into a single object, or write an object to multiple files.

* Instead of reading a file into an object, you might want to stream the file using a generator (e.g., if the file cannot be fully loaded into memory).

To address these issues (and others), scikit-bio provides a simple, powerful interface for dealing with I/O.