chipKIT-core v2.0.0 release

This release of chipKIT-core replaces our original USB stack with a brand new one, written from scratch by @majenkotech which not only works on both MX and MZ families of PIC32 parts, but also allows more than just USB serial emulation (keyboard, mouse, etc. is available.)

This version also includes pic32prog v2.1.37 which fixes a major bug when writing blocks of Flash that contain nothing but 0xFFs.

Other changes include updated I2C (wire) library with get/set clock calls and fixed return values as well as new clock speeds.

Release notes here : https://chipkit.net/wiki/index.php?title=ChipKIT_core_release_notes