Separate code into package 'taipei'

This is the first step towards making Taipei-Torrent available as a library. I've moved all of the files (besides `main.go`) into the `taipei` subdirectory, and changedthem to use the `taipei` package.

There are a lot of changes in the PR, because it's necessary to capitalize the names of all fields used in `main.go`, since none of them were exported. Later, we can decide what fields really should be exported.