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
* Vim-go will have version tagging,  beginning with "1.0.0"  so people now on which version they are. However this will introduced after fixing the Windows bugs #153 and #165.

The changes are open to discussion albeit I'm strict on same cases such as autoinstall binaries or gofmt. I'm curious about the impact and any comment is welcome. I'm trying to my best to avoid huge changes.