Refactored and splitted crypto module into several modules

The structured directoy layout is the following:

* ciphers are now located in sys/crypto/chipers/*
* hashes are now located in sys/crypto/hashes/*

Each cipher (AES, 3DES, RC5, SKIPJACK, TWOFISH) / hash (sha256)
can be used with USEMODULE-Macro.
Apply coding styles to crypto ciphers and hashes
Added NULL cipher for crypto operations
Added generic interface for block ciphers

* cipher_init(), cipher_set_key(), cipher_get_block_size()
* cipher_encrypt(), cipher_decrypt()
Added unittests for AES crypto cipher

As test vector we used input/output of PyCrypto 2.6.1:

  from Crypto.Cipher import AES
  binascii.hexlify(AES.new(key, AES.MODE_ECB).encrypt(msg))
Added unittests for 3DES crypto cipher

As test vector we used input/output of PyCrypto 2.6.1:

  from Crypto.Cipher import DES3
  binascii.hexlify(DES3.new(key, DES.MODE_ECB).encrypt(msg))
Added unittests for Twofish crypto cipher

As input/output for the test vector we used Twofish 0.3.0

  from twofish import Twofish
  Twofish(key).encrypt(msg)
Added block cipher mode operation support

The following cipher modes are now supported:
* Electronic code book (ECB)
* Cipher block chaining (CBC)
* Counter (CTR)
* Counter with CBC-MAC (CCM)

All modes are included in the 'cipher_modes' Module.
Added unittests for cipher API
Added unittests for cipher operation mode CCM

Test vectors are taken from RFC3610 page 11ff
Added unittests for cipher operation mode ECB

Test vectors are taken from 'Recommendation for Block Cipher Modes
 of Operation - Methods and Techniques' by Morris Dworkin (NIST)
Added unittests for cipher operation mode CBC

Test vectors are taken from 'Recommendation for Block Cipher Modes
 of Operation - Methods and Techniques' by Morris Dworkin (NIST)
Added unittests for cipher operation mode CTR

Test vectors are taken from 'Recommendation for Block Cipher Modes
 of Operation - Methods and Techniques' by Morris Dworkin (NIST)
Improve doxygen documentation of RIOT's crypto system