Add support for Teensy development platform

[ivankravets]

Tasks:

• Create new development platform named teensy by @PaulStoffregen, including supported boards (Teensy 2 & 3) and build scripts (Assigned to @valeros)
• Compile teensy_loader_cli for Linux 32/64 + ARMv6l (Assigned to @valeros)
• Prepare framework-arduinoteensy and tool-teensy packages (Assigned to @valeros)
• Upload dependent/updated packages to SF (Assigned to: @ivankravets)
• Test uploading to Teensy 3.1 on Mac, Windows, Linux 32/64
• Add information about teensy platform to main documentation (Assigned to: @ivankravets)
• Add information about teensy platform to PlatformIO Web Site (Assigned to: @ivankravets)
• Ask people to test it on PJRC Forum (Assigned to: @ivankravets)