Convert to SoftwareSerial

[cterwilliger]

Convert library to SoftwareSerial so ESP8266 can use native USB port for debug. Using D7/D8 for rec/xmit. Also added NULL pointer checks that were previously mentioned by others. Assuming some may want to stay with HardwareSerial, so maybe this branch can just sit here for whoever wants it. Modified .cpp, .h, and example files. Tested on ESP8266 NodeMCU 1.0 (ESP-12E).

[winginitau]

Gday! Thanks heaps for working on this. Would be great to merge it into the master.

Perhaps you could add some board directives:

// #define ESP
#ifdef ESP
...
#else 
...
#endif