Added custom navdata messages and message generator
NOT FINISHED: Incorporating custom messages into code

Sorry for the commit, I wanted to switch back to the main branch for a
while
Revert "NOT FINISHED: Incorporating custom messages into code"

This reverts commit f46a7adf8985f6e177dea9e6945363a896643b5e.
Added custom messages and handling code

We now have the ability to access any of the navdata structs which are
sent back by the drone. Still to do is to enable/disable the drone
sending based on whether we're interested in receiving the message.
Enabling all messages

Still to do is add a flag to disable the default navdata and imu
messages
Added the ability to disable hover mode and constantly send commands

As per parcon's suggestion, I'm adding the ability to disable the hover
mode to allow for more predictable and thus modellable drone dynamics.
This should also be useful for my purposes.
Added enable_legacy_navdata (true by default)

This flag allows us to turn off the legacy navdata/imu/mag topics,
favouring usage of the actual packets
No more cache stuff!
Merge branch 'more-navdata'
Updated to include some new flags and enable more aggressive flight

This is an aggressive launch file, you should turn down the
euler_angle_max, control_vz_max and control_yaw if you are not
interested in aggressive flight!
Moved launch file to an aggressive launch
Merge branch 'master' of https://github.com/mikehamer/ardrone_autonomy into mikehamer-master
Restoring sample launch file.
Minor bug that prevented build on groovy, fixes #35 @overbit
Some comments added for future reference.
cmd_vel.angular.x/y != 0 will disable hover mode

Removed ros params command_disable_hover and command_always_send, in
favour of disabling hover manually by setting cmd_vel.angular.x=1 or
cmd_vel.angular.y=1
navdata_<packet>.header is now correctly filled

Header.stamp and header.frame_id are now correctly filled. Messages are
also now member variables rather than local to save initialization time.
Merge pull request #38 from mikehamer/master

navdata_packet.header now filled, also using cmd_vel.angular.x/y != 0 for hover
Fixed bug in auto hover preventation #38. Also reversed the logic for
`hover` variable to make the logic correct.
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