This resolves #149

This is partial progress on #149

Changing calls to now() to always use a node reference instead of instantiating a clock object (#172)

* Changing now() to use the node clock in DWB

* Fixing clock usages to use node clock in costmap, amcl and map server.

Factoring out duplicated initialization code and addressing ROS naming convention violation

AMCL Readme

First draft of ROS Comparison doc

Fixed typo 'an' Task -> 'a' Task

Changed wording from "ROS Navigation" to "ROS1 Navigation"

* Update map_server README
* added issue

* First cut at the Simple Navigatgor README

Fixed few typos.

More clearly called out nav2_bt_navigator and fixed typo

Adding unit test to robot class and disable parsing URDF file

Ability to build Pull Requests in Travis

This is a a workaround. The current AMCL code frequently fails to
transform data and generate a pose. The current hypothesis is that
this is a result of the message filter code having been removed.

This change works around the problem by adding a 1 second timeout
on each transform, meaning there is now a 1 second window from
when the data is received till the transform can become available.

It's not clear whether this workaround has any unintended
consequences. Specfically I'm concerned about performance and
latency.

Parse URDF file from ros_robot class