Skip directories when bootstrapping.

Fixes #4.
Move directory check into compile().

This covers both bootstrap and regular (watching) mode.

Also includes check if the filename has a ".dust" extension.

Fixes #4.
Preserve relative path of dust templates.

Fixes #4.
Normalize paths before operating on them.
Merge pull request #7 from ZoogieZork/ignore-dirs

Support nested template directories.