Fixes to make 0.2.0-beta work with ESP32

[brunokc]

[It may be cleaner to compare with the 0.2.0-beta tag in order to see the actual changes]

These were the changes I needed to do in order for the libary to compile and work in PlatformIO.

• PlatformIO Core 3.6.5
• ArduinoJson 6.9.1
• ExpressIf 32 1.7

Summary of changes:

• Dropped paged-pool.*
• Now using the general WebServer instead of specific ESP8266WebServer one
• Fixed Rest-Esp8266.h:
  • Renamed to Rest-Esp.h to accomodate both ESP8266 and ESP32.
  • Fixed DynamicJsonDocument constructor, now specify some value for initial allocations
  • Fixed RequestHandler::handle() which was ignoring the return value of the handler
• Missing #include and (using string.h for memset() instead of memory.h as it was originally -- for some reason memory.h is not available in my environment)

[brunokc]

As mentioned before this branch was created on top of the 0.2.0-beta tag so it probably won't merge clean against master as this PR has been set up as (it may be easier to compare against 0.2.0-beta tag to get a clean diff).