bugfix: avoid retrying the build due to bootstrap when it fails
adding external plugin sample
first iteration of external plugin capability

This big fat commit enables the use of external pypi plugins as
seen in #61.

Plugins are referenced with use_plugin("pypi:my_plugin_name").

This requires the following to work:

  * There must be a package "my-plugin-name" on PyPI
    (Due to python package naming conventions, pip replaces
     underscores with dashes. So the package name MUST be
     "my-plugin-name").

  * After installing the package "my-plugin-name", it must be
    possible to import it with "python -c import "my_plugin_name".
    The package must expose its PyBuilder interface (tasks, ...)
    in its __init__.py (either declared there or imported).

  * NOTE: It is possible to circumvent the naming hitch by using
          a package name without underscores entirely (dashes are invalid).
add example plugin to our build.py
shorten line
removed superfluous newline
Merge pull request #77 from pybuilder/feature/external-plugins

Feature/external plugins