Paths in nbexchange plugin

[lzach]

Nbexchange inherits some of the path handling from nbgrader, including some of the issues.

One solution to fix this is to replace the current way having every class constructing the paths themselves with dedicated methods that do all the path handling in one place.

Another improvement is to replace the use of regex and globs to list paths (as it breaks easily when needing to regex-escape input), with functions that walk the specified path, picking up all the user and assignment details as it goes along.

This is closely related to the #14 issue, and a branch exist that tries to implement these ideas.