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/trees/binaryheap/serialization.go

@@ -0,0 +1,33 @@
+// 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 binaryheap
+
+import (
+	"github.com/emirpasic/gods/containers"
+)
+
+// Assert Serialization implementation
+var _ containers.JSONSerializer = (*Heap)(nil)
+var _ containers.JSONDeserializer = (*Heap)(nil)
+
+// ToJSON outputs the JSON representation of the heap.
+func (heap *Heap) ToJSON() ([]byte, error) {
+	return heap.list.ToJSON()
+}
+
+// FromJSON populates the heap from the input JSON representation.
+func (heap *Heap) FromJSON(data []byte) error {
+	return heap.list.FromJSON(data)
+}
+
+// UnmarshalJSON @implements json.Unmarshaler
+func (heap *Heap) UnmarshalJSON(bytes []byte) error {
+	return heap.FromJSON(bytes)
+}
+
+// MarshalJSON @implements json.Marshaler
+func (heap *Heap) MarshalJSON() ([]byte, error) {
+	return heap.ToJSON()
+}