Refactored crypto module into structured directoy layout

* crypto ciphers now located in sys/crypto/chipers/*
* crypto hashes now located in sys/crypto/hashes/*
Added NULL cipher and 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)
Fix undefined references issues for pow

Linker (for gcc < 4.9) is complaining about missing math functions
(like pow) even though we have -lm in "Makefile.include". If we put
"-lm" as last statement it just works.
Added unittests for crypto aes operations
Renamed ciphers from MODULE_CIPHERS_<name> to MODULE_<ciphername>