changed parameters form `kernel_pid_t` to `int`
core: priority_queue: simplify inheritance
vtimer: no needless type casting

Compare https://github.com/RIOT-OS/RIOT/pull/1639#issuecomment-54713048
and following discussion.
Merge pull request #1644 from Kijewski/vtimer-less-casting

vtimer: no needless type casting
Merge pull request #1639 from authmillenon/simplify_queue_inheritance

core: priority_queue: simplify inheritance
sixlowpan: ignore incoming packets if unconfigured

If there's no IPv6 address configured so far to any interface, 6lowpan
should not try to  handle incoming packets. This can easily lead to
looping packets.
sixlowpan: documented internal is_our_address()
Merge pull request #1606 from OlegHahm/sixlowpan_ignore_unconfigured

sixlowpan: ignore incoming packets if unconfigured
tests: don't build test/libfixmath on Travis CI

Very often the build unexplainably hangs.
Merge pull request #1648 from Kijewski/no-test-libfixmath

tests: don't build test/libfixmath on Travis CI
Merge pull request #1561 from BytesGalore/fix_fd_parameter_warnings

sys/posix: changed parameters for `fd` from `kernel_pid_t` to `int`
transport_layer: Splitting UDP and TCP

Currently, the tcp and udp implementations are bound to each other in a
module called *destiny*. Thus, when using only one of them then the
other one gets also compiled into the binary and initialized,
which results in unnecessary RAM usage and workload for the CPU.

The approach in this PR defines a common module named *socket_base*,
which contains functions used by the posix layer. Compiled by it's own,
those functions return negative error codes, to symbolize upper layers
that they are not supported. When also including the modules *udp* or
*tcp* respectively, functions from *socket_base* get overwritten with the
correct functionality.

Defining *udp* or *tcp* in a Makefile also includes *socket_base*.
Defining *pnet* in a Makefile also includes *socket_base*.
Merge pull request #1508 from cgundogan/transport_layer_refactoring

transport_layer: Splitting UDP and TCP
atmega_common: add ENOTSUP and ECANCELED to errno.h
drivers: Provide basic interface for all network devices
drivers: Make general IEEE 802.15.4 driver part of net_dev
nativenet: Apply coding conventions
nativenet: Supply net_dev_t support for nativenet
net_dev: Provide some default values dependent on module selection
unittests: add test network device

The thinking behind this commit is twofold:
1. test the "lower" part (the device part) of net_dev
2. provide a well defined testing infrastructure for modules that utilise
   net_dev
The actual tests are provided in the following commit