WIP: Added support for ESP8266

[whs]

Hi!

After finding out that riverscn's fork was reading data at incorrect size, I decided to attempt on adding SoftwareSerial support to this library to support ESP8266

So far it build successfully, but it won't link. I'm not proficient with C++ so I'm posting here in case anyone would know how to fix this:

/home/whs/.arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/2.5.0-3-20ed2b9/bin/../lib/gcc/xtensa-lx106-elf/4.8.2/../../../../xtensa-lx106-elf/bin/ld: sketch/weatherstation.ino.cpp.o:(.text._ZN17PmsSoftwareSerialD2Ev[PmsSoftwareSerial::~PmsSoftwareSerial()]+0x0): undefined reference to `vtable for IPmsSerial'

If I comment out the destructor in IPmsSerial then it build just fine, but I feel like that would break something else...