Make pluginPath plural and a list.

This makes it easier for people to setup multiple plugin paths. Also add
the skeleton for the viewPaths as App::build() is going away soon.
Remove App::build() and App::paths().

Replace App::build() with a configure value for view paths. Since view
paths are generally not modified a configure value should be enough.
Also maintaining the old method name when it does very little of what it
used to do is misleading.

Update App::path() to read from the new configure values and provide
a nice interface for developers to use.