Add spellchecking tool

This is similar to what's currently in the Makefile, but it does a little
bit more.

It's quite flaky, like everything I put in the tools directory. But
that's ok because it doesn't run anywhere important and it finds
typos, which is better than not finding typos.

The number on the 1st line of the dictionary file is the number of
words in the dictionary. I think it's approximate so that aspell
knows what kind of size of memory it's going to be dealing with.
Add Travis file

Run spellchecking as part of the build process.

It's not mature enough to fail the build, but it at least gives
us a link we can click to review a short list of words. This gives
us a much higher chance of spotting misspellings.