pull some content and definitions into a new "Getting Started" chapter

[gregcaporaso]

Pull some ideas/concepts/definitions from the individual chapter to a new Getting Started chapter would allow students to jump around more to focus on the chapters they're interested in. For example, defining pairwise alignments would let students read the Sequence Homology Searching chapter before the pairwise alignment chapter (which I experimented with in class this year - it worked well, but we need this bit of context). Multiple sequence alignments and trees would also be good to define in this section, and these could be referred back to later. I think the content mostly exists to achieve this, it just needs to be moved around.