docs: Get API docs working again

This changes around a few things that didn't work for me:
* Section names seem to be ostree-* instead of libostree-*
* Also XML files are ostree-* (they didn't show up at all)
- gtk-doc doesn't seem to parse const _OSTREE_PUBLIC correctly
* pull documentation is now on the actual functions rather than stubs
* Update gitignore with some more files

And there some changes to make gtk-doc give fewer warnings (not finished)

Closes: #327
Approved by: cgwalters

src/libostree/ostree-diff.h

@@ -27,11 +27,17 @@
 
 G_BEGIN_DECLS
 
+/**
+ * OstreeDiffFlags:
+ */
 typedef enum {
   OSTREE_DIFF_FLAGS_NONE = 0,
   OSTREE_DIFF_FLAGS_IGNORE_XATTRS = (1 << 0)
 } OstreeDiffFlags;
 
+/**
+ * OstreeDiffItem:
+ */
 typedef struct _OstreeDiffItem OstreeDiffItem;
 struct _OstreeDiffItem
 {