Go 1.5 released

This release includes significant changes to the implementation. The compiler tool chain was translated from C to Go, removing the last vestiges of C code from the Go code base. The garbage collector was completely redesigned, yielding a dramatic reduction in garbage collection pause times. Related improvements to the scheduler allowed us to change the default GOMAXPROCS value (the number of concurrently executing goroutines) from 1 to the number of logical CPUs. Changes to the linker enable distributing Go packages as shared libraries to link into Go programs, and building Go packages into archives or shared libraries that may be linked into or loaded by C programs (design doc).