core/net: Added support for "bridge mode".

"Bridge mode" allows devices to more easily send 802.15.4 packets as if
they were a different device. It also turns off any packet filtering
that may be implemented at layer 2. It works by allowing
`PACKETBUF_ADDR_SENDER` to be set earlier in the stack.

This is useful for implementing 6LoWPAN-ethernet bridges.

Enabled via setting `NETSTACK_CONF_BRIDGE_MODE` to 1. Disabled by
default.
Add mesh_ready() to mesh
mesh_ready checkes whether a packet is queued. This allows to avoid
packet loss at application level.
Split the collect test into collect and collect-lossy to make each individual
travis build complete faster.

Also changed the armgcc download link to a github location.
Updated paths to relative paths to make it easier to move tests to new directories
Let the framer functions use ints instead of uint8_t to make them more generic
A number of updates to nullrdc

* add a few rimestats to keep track of sent and received acks
* made a number of configuration options possible to override (ack timing)
* added the logic for sending 802.15.4 link layer ack packets, despite not being able to guarentee the 802.15.4 MAC timing
* increased the number of sequence numbers to keep track of for duplicate filtering
Send announcements only if the node is configured to be a router
New mechanism for computing the 6lowpan header length dynamically, depending on how large header the framer creates. Also check the incoming packet for buffer overflow.
Rimestats bugfix: only compile the rimestats printout if rimestats are enabled
Minor code style fixes to the contikimac and nullrdc code
Bugfix: keep track of receiver being awake as a local variable rather than a global, as it may have changed between invokations
Defensive programming: make sure that we don't fail completely if we get a callback for a NULL pointer
Minor bugfix: explicitly cast pointer argument in debug printf
Include the rimestats variable only if rimestats are enabled by the configuration
Bugfix: check the size of the outgoing packet to avoid overflowing the output buffer
Merge pull request #75 from cmorty/mesh_ready

Add mesh_ready() to mesh
Merge pull request #135 from darconeous/pull-requests/core-net-bridge-mode

core/net: Added support for "bridge mode".
Major bugfix: when nullrdc is asked to send a list of packets, send the list of packets and not just the first packet - this is essential for retransmissions of fragmented IP packets to work
Merge pull request #172 from adamdunkels/feature-split-collect-regression-test

Regression tests: split the collect test into two, moved armgcc download location
Merge pull request #176 from adamdunkels/feature-net-bugfixes

Network system bugfixes