Support Teensy 3.0 & ARM, plus minor const pointer fix

This pull request has 3 types of changes.

#1: Workarounds for conflicts with newlib C library, used on ARM-based boards (2 commits)
#2: Add Teensy to interrupt pin mapping array
#3: Fix pointers to constant strings with "const" qualifier

These are made in separate commits.

These changes have been verified to work, and they have also been verified on an Arduino Uno board to not break the library for normal AVR-based Arduino.

http://forum.pjrc.com/threads/23954-WiFi-module-with-an-SPI-interface-project?p=34906&viewfull=1#post34906