Make handling of different manifest names, and rev-suffix cleaner

- move where check for if it is a manifest
- error when manifest file is not a json object
- jslinted as well

my answer to #5 ; as there is advantages to using gulp-rev-collector (due to memory and stack issues with rev sometimes).

This could potentially still be improved, but I think it is a step in the right direction (and is generally backwards compatible).
This function is now only ran when it IS a manifest file:
```javascript
if (path.basename(file.path) === opts.manifestFilename) {
    _.extend(manifest, _getManifestData(file, opts.manifestFilename));
} ...
```