Merged lots of PR + fix some memory leaks

Library did some allocs without free and lots of un-initialized variables.

Applied pull requests from esp8266-oled-ssd1306:

squix78/esp8266-oled-ssd1306#169
squix78/esp8266-oled-ssd1306#157
squix78/esp8266-oled-ssd1306#144

And applied the suggestions from issues:
squix78/esp8266-oled-ssd1306#170
squix78/esp8266-oled-ssd1306#161

Support multiple display resolutions

Applied PR from esp8266-oled-ssd1306:
squix78/esp8266-oled-ssd1306#133

Better Brightness / Contrast control

Suggestions taken from this issue:
squix78/esp8266-oled-ssd1306#134

It is now the same version as being used in ESPeasy.