Add defer for Unlock mutex

* Refactored mu.Unlock lines and added defer statement for more safety.

Signed-off-by: Gökhan Özeloğlu <gozeloglu@gmail.com>

Update README

Signed-off-by: Gökhan Özeloğlu <gozeloglu@gmail.com>

Add doc.go

Signed-off-by: Gökhan Özeloğlu <gozeloglu@gmail.com>

Refactor item Key

Item.Key type converted from string to interface{}. Functions parameter and
returns types changed to interface{}.

Signed-off-by: Gökhan Özeloğlu <gozeloglu@gmail.com>

Add Cap() function

All lines that uses Cap are refactored with Cap() function.
Tests added for Cap().
README and main.go updated.

Signed-off-by: Gökhan Özeloğlu <gozeloglu@gmail.com>

Add Len() function

All lines that uses Len are refactored with Len() function.
Test added for Len().
README and main.go updated

Signed-off-by: Gökhan Özeloğlu <gozeloglu@gmail.com>

Add Peek() example

Example code added in main.go and README

Signed-off-by: Gökhan Özeloğlu <gozeloglu@gmail.com>

Update documentation

Signed-off-by: Gökhan Özeloğlu <gozeloglu@gmail.com>