Add additional documentation in core::io.

Added docs for stdout, stderr, print, and println.
Add docs for stdin in core::io.
Added note about prelude inclusion.
auto merge of #6620 : steveklabnik/rust/stdin_docs, r=brson
Fix trailing whitespace
Reexport static trait methods on traits in the same module.
Remove unused variable.
Explain an odd conditional check.
auto merge of #6432 : thomaslee/rust/issue-4202-02, r=catamorphism

My earlier fix for #4202 would not work correctly if the trait being exported was a top-level item relative to the module from which it was being exported. An example that would not work correctly with the original patch:

    // foo.rs

    pub use Foo = self::Bar;

    pub trait Bar {
      pub fn bar() -> Self;
    }

    impl Bar for int {
      pub fn bar() -> int { 42 }
    }

    // bar.rs

    fn main() {
      foo::Foo::bar()
    }

This is now supported.

I believe this change will allow the GenericPath trait to be exported from core::path as Path in such a manner, which should allow #5389 to move forward.
auto merge of #6619 : steveklabnik/rust/stdout_docs, r=thestinger

Added docs for stdout, stderr, print, and println.