Hideous but functional type generation using RobotOS.jl

[colinxs]

This PR is a proof-of-concept for issue #1.

The primary purpose is to:

1. Generate types that RobotOSData does not contain
2. Generate types that are compatible with RobotOS

A few things:

• Hideous and hacked implementation. Lot's of try/catches and eval(Meta.parse(...)) because I was lazy and needed a quick fix.
• RobotOS.rostypegen() rebuilds the whole package, so if a new type is found in a package
  that was already built then a "WARNING replacing module ...." is generated.
• For purpose (2) it would be nice if the type definitions in RobotOSData were compatible with RobotOS
  (e.x. publishing a RobotOSData.CommonMsgs.geometry_msgs.LaserScan in RobotOS). Towards that end, one option would be to scrap RobotOSData's msg definitions and add the ability for RobotOS's types to be written to a file (so that the types can be re-used on systems where ROS isn't available, which I believe was the motivation for RobotOSData's method of type generation).
• We could use Requires.jl to make this optional based on whether the user already has RobotOS/PyCall etc.
• This will fail Travis unless ROS is installed.

Thoughts?