Clean up tree enumeration and quick access API.

- Rename `-contents` to entries and make it a property.
- Rename `enumerateContentsWithOptions:` to
`enumerateEntriesWithOptions:`
- Switch block return type from `int` to `BOOL`
- Add a `BOOL` *stop parameter to the block.
- Reorder the block arguments so that the enumerated object is first to
be passed in.
- Add a test on `enumerateEntriesWithOptions:`, testing the *stop
parameter.
- Logic and Style fixes.
Add a space between block type and name.
Explicit comparison
Add test testing descendant behavior

As part of the addition reformat tests to use the `describe` function
Documentation fixes
Clarify what the root parameter does
Omit return type
Documentation is now under 80 chars

Fixes misspelling as well.
Clarify documentation
Add test for `GTTreeEnumerationOptionPost`
Merge pull request #316 from PiersonBro/tree-enumeration-cleanup

Clean up tree enumeration and `-contents` API.
Correct LICENSE link

Fixes #320.
Fix the rendered name of the file too
Merge pull request #321 from libgit2/cleanup-ballin-octo-nemeses

Correct LICENSE link