Create dev install script
Remove deprecated directory creation

Removing deprecated functionality in preparation for Sketch 3 support.
Rename preview finders

Rename preview finder functions to abstract them from Sketch
implementation names. As discovered by @TomasLinhart in #8, Sketch 3
changed the names of some messages. By generalizing the naming used in
the plugin, we should be able to support Sketch 2 and 3 with less
confusion, and we're better insulated from future Sketch changes.
Support Sketch 3

This change adds support for Sketch 3, building on the work done in #8.
The main difference is to use `respondsToSelector:` to send the
appropriate message based on what's available in the executing version
of Sketch. We also more gracefully handle the case where layer
collection messages disappear, so the subsequent search strategy will
still have a chance to run.

Thanks to @TomasLinhart for his work.
Update README for Sketch 3
Log debugging information

Add the ability to selectively log debug info. This should help
troubleshoot future problems with Artboard selection. It seems that in
the latest version of Sketch 2, a layer within an Artboard must be
selected for that Artboard to register as selected. I think this is a
new development, but I have no way to test it. Regardless Artboard
selection works as expected in Sketch 3.
Fix named slice search error handling

Returning false was the wrong thing to do, as the calling code would try
to iterate on that value, which is of course not possible. Now we return
an empty array and log a suitable error message.
Update Changelog
Rename Changelog to uppercase

It seemed like this should be consistent with the OTHER FILES.
Add troubleshooting section to README