Allowing to select the remote and the branch pulling from.

This is a first stab at fixing issue #435.
Fixed a few typos.
Removed an unused `format`.
Placed the options in the right place for `pull`.

Copying the code from `magit-push` was not possible because `magit-pull` is less flexible with its options.
For `magit-push` the options can be passed after the `repository` and the `refspec`.
However, at least for `--rebase`, `magit-pull` does not accept this format.
Merge commit '3062d83ea949d731c4c5b0558de94ec0e19ae9b7' into dudebout/issue435
Merge branch 'dudebout/issue435'