ESP Arduino framework fix

[Xanderr]

Fix for the issue discussed here: #3

There are some minor additional tweaks:

• Automatic call of SPI.begin() when you call the begin() function of the library.
• All examples have been updated to apply the fix (except for the wireless serial link and callback irq examples). I tested this on both Arduino and ESP, and works fine.
• Simple explanation added to the debug sketch on how to use a different SPI bus on ESP.

[hcoohb]

I can confirm this PR fixes the problem with esphome.
Thanks @Xanderr for putting this together!