Tree __contains__ checks for subdirectories

This is a really annoying inconsistency:

```
>>> "foo/boo" in tree
False
>>> tree["foo/boo"]
<_pygit2.TreeEntry object at 0x7f9c44a57318>
```

I can see why it happens but it's incredibly unnatural.

Similarly, it would be nice if we didn't have to create TreeBuilders for each subdirectory when creating blobs in a subdirectory. eg. be able to do something like `treebuilder.insert("foo/bar", ...)`
