Add support for building as ESP-IDF component

[coldfix]

Hi,

This change simplifies integrating RTClib as an component into an ESP-IDF project. With the change, all that has to be done is to clone RTClib + Adafruit_BusIO into the components/ subdirectory of the IDF project, and declare RTClib a dependency in the relevant CMakeLists.txt files like this:

idf_component_register(
  SRCS ...
  INCLUDE_DIRS ...
  REQUIRES RTClib
)

Resolves #290