Standardize file path access throughout ghost

resolves #1390

update all string based references to file paths
to use the ./core/server/config/paths file
so that it is the single source of truth
Merge pull request #1636 from hswolff/standardize-path-access

Standardize file path access throughout ghost
CasperJS waitForOpaque fails more gracefully
Serving static assets via asset-rack

closes #1405
- adds asset-rack to package for serving assets
- moves all assets previously served via express.static into an asset rack
- handles url generation for statically served assets
- updates the asset-helper for hbs-templates to get the right url
- allows configuration of local-asstets-deployment directory and CDN