Fix bug where docpress would not load external scripts

[victorquinn]

Due to the fact that docpress expected all scripts to be in the files array (consisting of local files only), it was not properly adding script tags for external scripts.

In fact providing an external url would cause docpress to crash with the following error since an external url would not be in the files object:

        if (!file.contents) return
                 ^

TypeError: Cannot read property 'contents' of undefined
    at scripts.map (/Users/victor/Documents/Development/chancejs/node_modules/docpress-base/index.js:134:18)
    at Array.map (native)
    at callback (/Users/victor/Documents/Development/chancejs/node_modules/docpress-base/index.js:128:33)
    at /Users/victor/Documents/Development/chancejs/node_modules/browserify/index.js:778:13
    at ConcatStream.<anonymous> (/Users/victor/Documents/Development/chancejs/node_modules/concat-stream/index.js:36:43)
    at emitNone (events.js:91:20)
    at ConcatStream.emit (events.js:185:7)
    at finishMaybe (/Users/victor/Documents/Development/chancejs/node_modules/concat-stream/node_modules/readable-stream/lib/_stream_writable.js:475:14)
    at endWritable (/Users/victor/Documents/Development/chancejs/node_modules/concat-stream/node_modules/readable-stream/lib/_stream_writable.js:485:3)
    at ConcatStream.Writable.end (/Users/victor/Documents/Development/chancejs/node_modules/concat-stream/node_modules/readable-stream/lib/_stream_writable.js:455:41)

This fixes the bug to ensure that Docpress will add a script tag for a scripts at an external url and that it no longer crashes when provided such a url.

[knownasilya]

no new required here.

[victorquinn]

Whoops good catch!

I first tried another module then moved to the core node one to prevent an additional dependency and accidentally left this.

Will fix as soon as I'm able!

[coveralls]

Coverage decreased (-1.2%) to 81.152% when pulling adf5f66 on victorquinn:master into dd0e548 on docpress:master.

[knownasilya]

I've released the latest changes as v0.7.3