Add inplace option to DataFrame.update()
Currently `DataFrame.update()` modifies the dataframe in place. I feel this is not symmetric to how most other methods work, e.g. `set_index()`, that returns a new `DataFrame`. It is probably too late to change this default behavior (isi it?), but if at the very least an `inplace` option was added (with default value `True` to support the current behaviour), it will be easier to chain a sequence of operators that includes `update()`.
