Easy calibration

[Psychokiller1888]

Added a calibration page for the ESP8266 module

The calibration page supports relative offsets, rather than absolute ones which would have required a complete reimplementation of WriteInstinct/OpenCat.h into the ESP module to access joints values, which wasn't acceptable in my opinion. To achieve the relative offset settings, I modified OpenCat main sketch to add the support:

• Sending a C command with a value of 1000 or higher will increase the offset by Value - 1000. Example: C12 1005 will increase the offset by 5 degrees from current position
• Sending a C command with a value of -1000 or lower will decrease the offset by Value + 1000. Example: C12 -1005 will decrease the offset by 5 degrees from current position