adding promise to all the methods, just let the callback be undefined…

[karianpour]

• Adding promise to all the methods, just let the callback be undefined to return a promise.
• Code cleanup.
• README update on executeCommand function.
• checkSimMemory promise bug fix, the callback fires with only one argument, so the first one should be result and the second error, this let us to remove unnecessary callback definitions.
• readSMSBuId callback was never used, so it is removed.
• initializeModem promise bug fix, the callback is being fired with one argument, so the first one should be result.
• for open and close methods, I suggest to change the callback arguments order to be (result, error) in order to be like the others.
• If sendSMS method is in promise mode, fires callback only after the sms is sent or failed, but for the sake of backward compatibility it is fired 2 times if there is a callback function provided.
• as we provide a promise callback if it is undefined, there is no need to check it if the callback is present or not to fire it, it is always present.