sys: remove lib/hashtable.c

Quoting myself:

> The code of sys/lib/hashtable.c is very badly adapted for embedded
> systems. The used primes are huge, and the limit is entirely
> unrealistic. Why do we ship this file?

In #1564 we came to the conclusion that we don't need this code.

Closes #1564.
core: SCHED_PRIO_LEVELS==16 for every board

Closes #1399.

> Using a different value for SCHED_PRIO_LEVELS for 16 and 32 bit
platforms hurts portability, one thing that we heavily advertise about
RIOT. if you want to write a portable application, then you have to
assume the lower value.

This PR defaults `SCHED_PRIO_LEVELS` to 16 for every board.
Merge pull request #1583 from Kijewski/issue-1399

core: SCHED_PRIO_LEVELS==16 for every board
Merge pull request #1572 from Kijewski/issue-1564

sys: remove lib/hashtable.c
cpu: atmega_common: Initial import
cpu: atmega2560: Initial import
board: arduino-mega2560: Initial import
tests: blacklist board arduino-mega2560 for tests/examples that don't build
sys: net_help: inet_pton: add conditional include of sys/types.h for atmega2560:
    Without this test/unittests fails to build due to a missing declaration of
    ssize_t