Add some whitespace.
Use the system clipboard.

Running all the tests concurrently blows away the current clipboard.
Add xset or xclip as a linux dependency.
Fix #402.
Make travis install xclip.
Mock the clipboard for most of the tests.

Avoid concurrency issues.
Use %q over %s in the clipboard test.
Try xsel over xclip for clipboard support on Linux.
Test setting the clipboard twice.

Avoid passing the test by coincidence.
Fall back on a local clipboard if the system one is inaccessible.
Go back to xclip.
Merge pull request #417 from erbridge/system_clipboard

Use the system clipboard