Misc fixes.

[caternuson]

A couple of fixes here.

• Fix #23. The busy wait loop was causing WDT reset on ESP8266. Added a delay(0) in the loop to allow ESP8266's implementation to yield as needed. This way don't need any pre-processor logic.
• Fix #38. Was not able to reproduce this, but based on conversation in issue thread it was related to calling an unsupported option DC2 * in begin(). Moved that configuration to a new function setPrintDensity() and then don't call it by default. Resulting prints look fine without it.

Here's an example WDT reset just for records:

[caternuson]

ESP8266 testing for the record. Previously it would WDT timeout on the bitmap print.