Improved handling of opts.paths

[Resistance]

• opts.paths are always preferred over bowerrc/defaults
• Introduced opts.paths.cwd; opts.paths='str' is shorthand for opts.paths.cwd='str'
• Using cwd from bowerrc only if present
• Using directory from bowerrc only if present (fixed a bug where missing directory was causing error to be thrown)
• Delayed resolution of opts.paths.bowerJson and opts.paths.bowerDirectory after opts.paths.cwd is resolved. Unfortunately bowerrc needs to be resolved before cwd, since it can specify both cwd and directory. This is only to keep backward compatibility with opts.paths='str'
• Appending path.sep to opts.base like gulp.src with globs would

[ck86]

Hey @Resistance,

• first of all, I will not merge because the tests failed.
• opts.path will be overridden if bowerrc is present, so it is not preferred
• there is only one bug I can see: the cwd option in the bowerrc file is currently ignored

The plugin just needs to know the path to the directory where the bower packages are installed and where is the bower.json file. So if the plugin does not find the right place without any given options, then you have 3 options to tell the plugin where to find the necessary files. Keep it simple.