Clean and separate Glob and ERB processing

I was able to split the importer into 2 clear concerns, Globbing and ERB.

Sprockets API-wise I was able to remove most of the deprecated or private calls. Pretty close to being able to allow for Sprockets 3.x!

The nice big change is that the importer is completely hands off if the import is neither a glob or erb file. This avoids the overhead for the case of a simple import. Those calls just do a `super`.

I'd still love it if we could get globs in sass core somehow. That at least remove all the bad dependency tracking hacks here.

/cc @rafaelfranca @matthewd 
The current glob implementation liberally accepts even broken glob syntax.

``` scss
# tested and pretty much works
@import "mixins/*"
@import "mixins/**/*"

# would be accepted but buggy
@import "{user,projects}/mixins/**/*"
# actually would just crash because it didn't have a "*" to force "glob mode"
@import "{user,projects}/mixins/variables"
```

None of this is actually tested or documented, so I think we're on pretty good grounds to remove support for it in `master.` I feel like we should only support globs that end in `/*` or `/**/*`. This mirrors sprockets' `require_directory` and `require_tree`.

I can propose this in a follow up PR.