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.
periph: added a random-data driver interface
Merge pull request #939 from mehlis/random-driver

random: added a random driver interface
Merge pull request #1583 from Kijewski/issue-1399

core: SCHED_PRIO_LEVELS==16 for every board
tests: limit concurrency level

Some Travis CI machines have 32 CPUs. This sets our concurrency level to 33.
Travis CI kills our buildtest for obvious reasons.

This PR limits the concurrency level to 8 on Travis CI.
cpu/sam3x8e: added random number generator driver
boards/arduino-due: enabled RNG driver
test/periph_random: added test for RNG driver
Merge pull request #1572 from Kijewski/issue-1564

sys: remove lib/hashtable.c
Merge pull request #1593 from Kijewski/nproc-max

tests: limit concurrency level
Merge pull request #1294 from haukepetersen/add_arduinodue_rng_driver

cpu: Add random number generator driver for the SAM3X8E
core: clist is a cyclic list
core: clist without explicit thread

Right now the core component `clist` is a generic cyclic doubly-linked list.
In the core it is used in `tcb_t::rq_entry`.
Further it is used `net_if.c`.

This commit removes the member `clist_node_t::data` which stored the
pointer to the `tcb_t` instance of which the clist is already a member.
The needless member added `sizeof (int)` bytes to every instance of
`tcb_t`.

In `net_if.c` the clist was used in a type-punned way, so that the
change won't affect it.