Add create_tree method for Index

[t3rm1n4l]

Invoke tree = index.create_tree() to create a tree from index file and return Tree object.

[jdavid]

This is cool, I need this feature very much.

Some remarks:

• PyObject_New will return NULL on error, handle this case
• git_tree_lookup may also return an error code. Well, maybe not in this particular case,
  (did not check the implementation), but it would be safer to check the error code anyway.
• there is a reference counting error, refcount must be increased for the repo,
  add the line "Py_INCREF(self->repo);"
• coding style: there is a mix of spaces and tabs, use 4 spaces for each indentation
  level, both in the C and Python files
• coding style: there is whitespace at the end of a line in pygit2.c

Thank you very much.

[t3rm1n4l]

Thank you :)
I will be careful about your notes for the next commit.

[t3rm1n4l]

I will send another commit with corrections tommorrow.