Add support for the Gitignore API.

This pull request adds support for the [Gitignore API](http://developer.github.com/v3/gitignore/).

The `GitignoresService` can be accessed with `client.Gitignores`. There are two functions available, `Gitignores.List` and `Gitignores.Get`. Everything is fully documented and tested.

Example usage:

```go
package main

import (
	"fmt"
	"github.com/zachlatta/go-github/github"
)

func main() {
	client := github.NewClient(nil)

	// List all available .gitignore templates.
	availableTemplates, _, err := client.Gitignores.List()
	if err != nil {
		fmt.Println(err)
	} else {
		fmt.Println(availableTemplates)
	}

	// Retrieve and print a specific template's source.
	template, _, err := client.Gitignores.Get("Go")
	if err != nil {
		fmt.Println(err)
	} else {
		fmt.Println(*template.Source)
	}
}
```