Adding Rake to automate the build, install, and uninstallation

Hello, 

I've added a rake tasks to automate the build system. This way, you don't need to open XCode just for installing the XVim plugin. Basically, it wraps the `xcodebuild` tool.

## Basic Usage

### Plugin Installation

```bash
$ rake
```
There will be an interactive prompt to choose Xcode version. Choose that, and the rake task will install base on the choice. To install directly

```bash
$ rake xcode56 # for XCode 5 or 6
```
The Rake script also do detection on the installation directory, if it already exists it'll prompt user to uninstall.

### Plugin Uninstallation

There's `uninstall` task which will uninstall the plugin

```bash
$ rake uninstall
```

Yea, I think it's fine. If they want to build XVim for Xcode4 they can do it manually.