Clean up build system

Switch to using go install. Leads to faster and cleaner builds.
go install directly installs the binaries into $VTROOT/bin directly
because $GOPATH==$VTROOT. It also intalls intermediate libraries under
pkg, which results in reuse.
There is also no more clutter in the source dirs: cleaner .gitignore.
Merge branch 'master' of github.com:youtube/vitess