Define the $wp_plugin_paths global array

This fixes the Invalid argument supplied for foreach() warnings generated by `plugin_basename()`. Related to wp-cli/wp-cli#1015
Add support for symlinking plugins if available

Adds support for the (new in 3.9) automatic detection and handling of symlinked plugins.

Related to wp-cli/wp-cli#1015
Merge pull request #1019 from johnpbloch/wp_plugin_paths

Add support for 3.9's handling of symlinked plugins