Additional points in your question:
** STOCK SPLITS & DIVIDENDS **
Regarding stock splits and dividends, the principle is to compare like for like. If you held 1 share at the start of the period, what would you be holding now?
If the stock has split, for example, so one share has become (say) 3 then you compare the price of one share in Dec'13 to the price of three shares in Dec '14.
If a dividend was paid during the period then the value in Dec'14 would be the value of shares + the value of the dividend.
Then compare the two values as per @d-stanley's answer.
** DIVIDEND RE-INVESTMENT ASSUMPTIONS **
One additional subtlety with dividends: you will have received cash during the couse of the year so we need to make an assumption about how the dividends were re-invested for the remainder of the period.
One common assumption is to assume it was re-invested in the same stock, so for example if received a 25p per share dividend at a time when the (ex-dividend) price was 500p (5% dividend yield) you could hypothetically buy one more share for each 20 you hold. In calculating the return you would compare 1 share in Dec'13 with (1+ dividend yield) shares in Dec'14.
In practice (unless dividend and equity returns are very high) there will not be a big difference in the shorter term between that assumption or just adding the cash value of dividends to the value of one share.
Hope that helps.