Import: Add support for absolute and ~ paths

Import paths can be in the format:

    relative/to/script.js
    /absolute/path/to/script.js
    ~/relative/to/home/script.js
import: reinstate warning for empty base in non-absolute URLs
Add support for nested @import statements

Also guards against multiple imports of the same URL, and therefore circular imports

Ref BohemianCoding/Sketch#3319
Fix a bug which ignored first line of @import-ed scripts
Merge pull request #21 from BohemianCoding/importabsolute

Add support for recursive imports, absolute paths