Implementation and tests for Trees API
rename TreesServe to GitService
misc cleanup of trees API

- complete the rename from TreesService to GitService.  For example,
  renaming Get to GetTree, and Create to CreateTree.

- rename GitTree to TreeEntry.  I really have no idea why GitHub chose
  to use the field name "tree" for this, since these aren't necessarily
  trees in the git sense.  Looking at it more closely, these are really
  just the entries within a tree, hence the rename.

- simplify several tests and shuffle some things for consistency with
  the rest of the library
Add Konrad to AUTHORS and CONTRIBUTORS file