Add support for .gitignore (#32)
* Add GitIgnore
* Rename *Mixin -> *MixIn
* Add {Project,Dir}.subpaths() to get an iterable of string subpaths
* Add {Dir,File}.parent to get the {Project,Dir} the object is contained within
* Update checklist and example in README
* Fix windows style separators (\) in GitIgnore
* Bump version to 1.6.0