Refactored into modules with core class

Renamed from mavlink* to mavbridge* to disambiguate.

Reworked command-line options; use '-h' for details.

Major gutting and refactoring to support extensibility:
* mavbridge_core.py contains a core class that blends rospy and pymavlink together and provides a very thin wrapper
* mavbridge.py provides a few "standard" ROS publishers and subscribers, and the command-line logic
* mavbridge_*.py are modules that can be loaded in with the '-m' option
Various fixes and feature adds

Fixes:
* Cleaned up custom mode enumeration in Status message
* Re-aligned enum dictionary in ACS module
* Simplified ROS subscriber callback wrapper
* Replaced some print's with exceptions

Features:
* Added ability to pass ROS publisher optional args in get_ros_pub()
* Added ability to pass ROS subscriber optional args in add_ros_sub_event()
* Added 'mode_num' topic to ACS module to use numeric modes from enum
* Added 'calpress' topic to ACS module to perform barometric callibration
Added support for ROS services, param module

Added add_ros_srv_event() to MAVLinkBridge class
Created module mavbridge_param.py to support getting/setting params
Created new service type Param.srv
Fixed type of heartbeat subscriber in 'acs' module
Param caching made optional, disabled by default

In some cases, the autopilot might not broadcast PARAM_VALUE
messages following a PARAM_SET from a controller. This means that
if a separate GCS instance changes a param, this software
won't find out without an explicit re-fetch. So, caching is disabled
by default and the module will re-fetch the param each time it
is needed. If caching is desired, it can be enabled in the param
module's init() function by passing True to use_cache in the class
constructor.
Added param_getlist service

Refactored some functions within the param module to support.

Also refactored service types to support this and to clean up
service type interface in general.
Added timed event support to core class

Introduced add_timed_event(self, hz, callback) to call 'callback'
at 'hz' Hz. Implemented in _mainloop(), so see caveats in the
comments.
Added waypoint (wp) module

New services:
* /wp_getall - Gets complete list of waypoints
* /wp_setall - Sets complete list of waypoints
Revised README