feat(installer): use go-git to clone

Integrate `go-git` into the application, and use it to `git clone`
cheatsheets when the installer runs.

Previously, the installer required that `git` be installed on the system
`PATH`, so this change has to big advantages:

1. It removes that system dependency on `git`
2. It paves the way for implementing the `--update` command

Additionally, `cheat` now performs a `--depth=1` clone when installing
cheatsheets, which should at least somewhat improve installation times
(especially on slow network connections).

vendor/github.com/emirpasic/gods/utils/utils.go

@@ -0,0 +1,47 @@
+// Copyright (c) 2015, Emir Pasic. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package utils provides common utility functions.
+//
+// Provided functionalities:
+// - sorting
+// - comparators
+package utils
+
+import (
+	"fmt"
+	"strconv"
+)
+
+// ToString converts a value to string.
+func ToString(value interface{}) string {
+	switch value := value.(type) {
+	case string:
+		return value
+	case int8:
+		return strconv.FormatInt(int64(value), 10)
+	case int16:
+		return strconv.FormatInt(int64(value), 10)
+	case int32:
+		return strconv.FormatInt(int64(value), 10)
+	case int64:
+		return strconv.FormatInt(value, 10)
+	case uint8:
+		return strconv.FormatUint(uint64(value), 10)
+	case uint16:
+		return strconv.FormatUint(uint64(value), 10)
+	case uint32:
+		return strconv.FormatUint(uint64(value), 10)
+	case uint64:
+		return strconv.FormatUint(value, 10)
+	case float32:
+		return strconv.FormatFloat(float64(value), 'g', -1, 64)
+	case float64:
+		return strconv.FormatFloat(value, 'g', -1, 64)
+	case bool:
+		return strconv.FormatBool(value)
+	default:
+		return fmt.Sprintf("%+v", value)
+	}
+}