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Merge pull request #17 from catkin/config

Config utilities for describing config files and CLI arguments

- new module `config_utils` for a general purpose tool to describe config files, parse 
  yaml config files and expose specific arguments to the command line; includes some
  unit tests for the config item types

- new module `config` describes xylem's global configuration; includes the logic to 
  compute sources and cache folders given command line arguments and config files

- new module `arguments` for global command line arguments (moved from `utils`); 
  interacts with config to include and process global config arguments

- new module `yaml_utils` contains customization to loading and dumping yaml (moved
  (from `utils`)

- `--help` argument now uses pydoc.pager to display the help in a `less`-like scrollable 
  manner on pty's

- help output includes new section `config arguments`, describing the arguments
  that correspond to a config file entry

- commands have a new hook to add to the config description; some logic common
  to all commands was moved to `commands.main`

- context objects like `InstallerContext` and `SourcesContext` now take a config
  dictionary as argument, which defaults to the global config singleton if not
  passed explicitly

- high-level API used by the commands (`update`, `resolve`, `lookup`) is also
  parameterized by an optional config dict argument, which defaults to the
  global config

- Introduce XylemError as common Exception base class. Having a common 
  base class allows users to catch all xylem related exceptions in one handler.
small update to maintenece utils
Update exemplar sources.

- sources point to custom branch of rosdistro containing converted rules files

- use the provided testing sources and cache by running from the directory of
  the xylem source checkout:

      xylem resolve boost --prefix prefix
Merge pull request #23 from catkin/utils

Small util fixes and updated example sources

- small tweaks to code maintenace utils
- sources point to custom branch of rosdistro containing converted rules files
- use the provided testing sources and cache by running from the directory of
  the xylem source checkout:

      xylem resolve boost --prefix prefix
Various smaller notes in design document.
update apidoc
fix broken reference in docs
Small coding related TODO document.
Minor notes and help output.
Move type_error_msg helper to exception module.
fix some rst syntax in docstrings
Refactor OS plugins.

- implement conclusions of latest discussions about derivative OSs and and core
  installers

- refactor using plugin_utils

- update unit tests
remove "priority" from rules dict
Move rules_dict to sources subpackage.
Remove all relative imports to parent packages (`..`)
Small tweaks to OS plugins.
Tweak type_error_msg utility.