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/containers/serialization.go

@@ -0,0 +1,21 @@
+// 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 containers
+
+// JSONSerializer provides JSON serialization
+type JSONSerializer interface {
+	// ToJSON outputs the JSON representation of containers's elements.
+	ToJSON() ([]byte, error)
+	// MarshalJSON @implements json.Marshaler
+	MarshalJSON() ([]byte, error)
+}
+
+// JSONDeserializer provides JSON deserialization
+type JSONDeserializer interface {
+	// FromJSON populates containers's elements from the input JSON representation.
+	FromJSON([]byte) error
+	// UnmarshalJSON @implements json.Unmarshaler
+	UnmarshalJSON([]byte) error
+}