Add top-level exception handler, unwrap AggregateException. Closes #6

Sometimes WebExceptions seem to be thrown as-is, while other times they
are wrapped in an AggregateException. Not entirely sure why, but now we
catch wrapped WebExceptions as well.

Additionally, with a new top-level exception handler the program
shouldn't outright crash anymore if an unhandled error occurs. Instead
it provides a pop-up that lets the user retrieve the error-message and
stacktrace that can then be included in a github issue.

Add option to download albums into separate folders.

Closes #5

For Reddit-downloads, albums may be saved into folders under the target
folder. The name of each album-folder is the name of the reddit-post.

For Local-filtering, this option preserves the file hierarchy of the
original folder in the target folder rather than just dumping the files
in the root of the target folder.

Imgur-albums cannot contain nested albums so this option is not supported
for Imgur-downloads.

Fix parallelism issue and bump version number

Also contains refactoring changes.

Fix auto-updater

After Github disabled TLS1.1 connections in february, the auto-updater has
been unable to get a connection to check for updates.

The HttpClient in .NET 4.5.2 defaults to TLS1.1; upgrading to .NET 4.6.2
upgrades the default protocol used by HttpClient.

Fix divide-by-zero when calculating aspect ratio

This happened when the default fallback width and height values of 0 were
used. Closes #4

Add tests for UpdateChecker logic

Initial commit