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
Merge remote-tracking branch 'upstream/master' into whird.jpope.org