remove db entries if file !exists

[keeferrourke]

imgrep init doesn't remove files from the db if they aren't found in subsequent scans of a directory

fix: keep file listing in memory when running files.Walker() and compare to db. ex:

fileListing := getFiles() // or whatever, just get a listing of the directory, k? string slice can be appended to as we run through Walker()
for _, f := 0 range file_listing {
     if !storage.IsInDB(f) {
          storage.Remove(f)
     }
}

or w/e

[keeferrourke]

related to #8

[keeferrourke]

Basically this issue is important to ensure we don't keep stale information hanging around.

[keeferrourke]

commit 30a593fed17f576a2a55b91fd8de4d714b116a5a fixes this issue; the db is cleaned up as results are returned to ensure that old/non-existent files do not appear in search results.