[tests] option to show warnings even if successfully compiled

Examples on how to test compile all aircrafts/targets in your current conf.xml:
with parallel compilation and showing full output during compilation
 $ J=AUTO prove tests/examples -v
only showing full compile output if there has been an error, if there were warnings only print those
 $ SHOW_WARNINGS_ONLY=1 prove test/examples
with parallel compilation and treating all warnings as errors:
 $ J=AUTO USER_CFLAGS=-Werror prove tests/examples
update travis config to show warnings
install libipc-run-perl in travis setup
[simulator] ignore stupid unused-parameter warnings in JSBSim
[nps] add missing include
[airborne] get rid of some const-qual warnings
Merge pull request #945 from flixr/tests_show_warnings_only

[tests] option to show warnings even if successfully compiled

Examples on how to test compile all aircrafts/targets in your current conf.xml:
with parallel compilation and showing full output during compilation
```
J=AUTO prove tests/examples -v
```
only showing full compile output if there has been an error, if there were warnings only print those
```
SHOW_WARNINGS_ONLY=1 prove test/examples
```
with parallel compilation and treating all warnings as errors:
```
J=AUTO USER_CFLAGS=-Werror prove tests/examples
```
[settings] fix multiple settings in modules

close #901
[build] remove 'load' target from makefile

It is a special command in Make 4.0
[build] remove unused load from phony list
[peripherals] add mpu9250 driver
[uart] Fix weak handler
[mpu9250] add test module and imu subsystem
[sensors] Added the AKM 8963 Magnetometer
[peripheral] fix some stuff in the ak8963 driver
[peripheral] update mpu9250 driver, imu and module as well
Merge pull request #953 from paparazzi/mpu9250

Driver for the MPU9250
[peripherals] mpu60x0: use gcc pragma to ignore cast-qual warnings

The buffer is volatile, since filled from ISR...
But we know it's ok to use it here so we silence the warning.

Also there is a bug in some gcc version that incorrectly reports this as
peripherals/mpu60x0_spi.c:118:33: warning: cast discards '__attribute__((noreturn))' qualifier from pointer target type [-Wcast-qual]
see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55383
[peripherals] ignore cast-qual for mpu9250 as well