Skip absolute path specifiers

[andrewiggins]

When passing a specifier such as /module.js to the resolve, resolve always tries to find the module at the root of the file system, not the root of the basedir. So I propose the node-resolve plugin skips absolute path specifiers since I think that trying to resolve for node_modules at the root of the file system is not very useful.

Since node-resolve will just pass along any specifiers it can't find, the only real behavior change that that I'm aware of is that node-resolve will no longer warn on absolute path specifiers when using the default logger.