Update tutorial, see issue #13578
auto merge of #14374 : swgillespie/rust/swgillespie-tutorial, r=alexcrichton

The current tutorial says that the only way to get master is to build from source, which isn't true anymore - nightly binaries and an installer for Mac OS X are now available at the install page: http://www.rust-lang.org/install.html . Feedback very much welcome! Addresses issue #13578.
Improve docs for core::tuple
auto merge of #14440 : Sawyer47/rust/tuple-doc, r=alexcrichton
std: Remove String::from_owned_str as it's redundant

[breaking-change]
std: Remove String's to_owned
std: Rename strbuf operations to string

[breaking-change]