Merge pull request #30 from jairot/master

Added Requirements.txt
Merge pull request #32 from jairot/master

created the new first version for the new plugin
add ctx.path_to_plugin_file function
add url_to_plugin_file to build url to fetch resource
add ctx.fetch_plugin_file
load plugins from default location and PBT_PLUGINS_PATH if set

first default location ($CONFIG_DIR/pbt/plugins) and if PBT_PLUGINS_PATH
is set as a colon separated list of dirs to look for look in those dirs
in order.
remove deprecation warning
add debug displaying where we are looking for plugins
convert PBT_PLUGINS_PATH to absolute paths
debug correctly
initial import of project.pbt for pbt
add yaml in dependencies
change old on_load function convention for pbt.run_on_load decorator
document plugins loading and basic structure
add instructions on testing
initial import of hello world plugin
show how to test a plugin