vim-go: several cleanups and behaviour changes

This is huge change to make vim-go compatible with all binaries and more in harmony with current Go tools. The changes are:

* Use `gofmt` by default instead of `goimports`. Unfortunately goimports is slow on large code bases.
* Introduce a new `GoImports` command that acts like `GoFmt` but uses under the hood the binary `goimports`. So users can now use still gofmt, but can execute `:GoImports` to automatically add/discard import paths.
* Disable automatically installing binaries at startup. Removing this is a hard decision but comes with several improvements. First not all users are using those binaries, therefore they shouldn't install and download them. It lowers the installation problems. Now vim-go can be used just for viewing Go files too without to download all binaries.
* However to make it easy (also added to README.md) I've introduced a new command called `GoInstallBinaries` and its companion `GoUpdateBinaries`. The first one installs all binaries to `$GOBIN` so vim-go users can easily make use of them. The second command is used to update those binaries if needed occasionally (for example to get the latest updates)
* Binaries no more get installed to `~/.vim-go/`. Instead it's using $GOBIN