Refactor template name logic into its own method

Having it in a separate method makes it easier to test and debug.
Some unit tests will be added to illustrate its behavior and highlight a
critical bug in the logic that makes the Dust plugin useless in a Windows
environment.
Add a few unit tests for DustTasks

They test the logic for getting a Dust template name from the file path.
There are two tests, for Unix and Windows respectively. The Unix one
passes, and the Windows one fails (deliberately).

The intention with the failing test is to illustrate a critical Windows
bug, which in effect makes it impossible to use the Dust plugin in a
Windows environment!

The bug is due to the fact that Windows uses backslashes for directory
separators, while the Dust plugin assumes forward slashes.

The next commit will make changes to the logic, to make the test pass and
fix the bug.
Fix failing test for DustTasks

The templateName method's behavior is changed. All slashes are now
converted to forward slashes first.
Merge pull request #54 from eirslett/master

Fix critical windows bug in the Dust plugin