update ament command to use verb pattern
update build_pkg to use verb pattern
added the concept of Context and ContextExtender
added concepts BuildType and BuildAction

As well as documentation about how to use them
to extend the supported build types for
ament_tools.
refactor build_pkg and the ament_cmake build type

They now use the BuildType and BuildAction API's.
temporary fix tests
refactor built-in build_types in ament_tools

- Made everything verb agnostic
- Changed the entry_point name
- Moved the source for the ament_cmake build_type
- Refactored discovery and loading of build_types
out of the build_pkg verb module.
fix order of option short circuits in ament cmd
build: refactor for verb and build_type patterns
list_packages: refactored to use verb pattern
package_name: refactor to use verb pattern
package_version: refactor to use verb pattern
test and test_pkg: refactor to use verb pattern

Also now uses build_type pattern.

Expanded ament_cmake plugin to include testing.
travis: use master branch of osrf_pycommon
style fixup
Merge pull request #4 from ament/use_verb_pattern

Use verb pattern