cbor: CBOR implementation for RIOT-OS

This is a malloc-free implementation of the Concise Binary Object
Representation (CBOR) data format for the RIOT-OS.

This implementation mostly stand-alone, and it should be pretty easy to
port to other platforms. We're only using the C STL and some custom
network-related functionaliy which could be easily replaced by depending
on arpa/inet.h.

The CBOR API is straight-forward to use and provides encoding/decoding
functionality for all major C types, such as:
- int
- uint64_t
- int64_t
- float
- double
- char*
- struct tm
- time_t

It is possible to conditionally compile this module via CFLAGS:
- CBOR_NO_SEMANTIC_TAGGING: All semantic-tagging features removed
- CBOR_NO_CTIME: All ctime related features removed
- CBOR_NO_FLOAT: All floating-point related features removed
- CBOR_NO_PRINT: All features depending on printf removed