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
Separated the uIP makefile from the top-level Makefile.include into a separate core/net/Makefile.uip makefile
Added #if UIP_CONF_IPV6 around the IPv6 code in the uIP source tree to allow the C files to be compiled even if IPv6 is not configured.
A default configuraion file for Contiki

The contiki-default-conf.h file is intended as a safe fallback for
a number of configuration options in Contiki, to avoid putting too
much in the individual contiki-conf.h files.
Changed the name of the IPv6 number of route entries configuration from UIP_CONF_DS6_ROUTE_NBU to UIP_CONF_MAX_ROUTES.
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