Relax lifetime bounds on Reader/Writer impls for trait boxes

Cargo needs this to be able to instantiate `TerminfoTerminal<Box<Writer+'a>>`
for 'a other than 'static.
auto merge of #16843 : bkoropoff/rust/reader-writer-box, r=alexcrichton

Cargo needs this to be able to instantiate `TerminfoTerminal<Box<Writer+'a>>` for 'a other than 'static.
Auto-merging 67b97ab

Travis: Passed at 2014-09-05T06:04:47Z
https://travis-ci.org/vhbit/rust/builds/34464108