add quotes around json tag values in order to be compatible with
reflect.StructTag.Get

The main purpose of adding this (aside from simply implementing the full
GitHub API) is to establish the basic Commit type, so that we can build
around it for things like Events (see #4) and WebHooks (see #20).

There shouldn't be a SHA in the URL
Also rename 'user' to 'owner'

- 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