Added scrolling capability

[agprimatic]

With my changes to Adafruit-GFX-library, Adafruit_SSD1306 now supports scrolling text. When text is written to the bottom of the screen and scrolling is enabled, the screen will scroll up one line to make room for the next text line.

[tablatronix]

Having a hard time understanding this pr, it looks like all you did was add doubling to y. What is the purpose of added function getpixel it does not seem related to this pr.

[agprimatic]

The getpixel function is needed in the Adafruit_GFX library, which is where the scrolling is actually done. Since I was submitting commits for both Adafruit_GFX and adafruit_SSD1306, I used the same title so people know they go together.

[tablatronix]

Doh just noticed its two prs my bad
should maybe add a ref link to these to an issue or the other pr.

[maxint-rd]

See also different implementation in PR #56 and adafruit/Adafruit-GFX-Library#132
(I prefer a flexible implementation in Adafruit-GFX that allows driver developers to choose between simple yet slow getPixel() scrolling, faster buffer manipulation scrolling, or superfast hardware scrolling.)