Add Signal device to docs
Add trigger method to Signal class
Test trigger method
Rename `fetch` command to `import`
Fix #304: add `export` command to concert script
Send image copy in downsize filter

which prevents memory leaks.
Use gevent only if wanted

before the `async` and `threaded` functions were implemented by
regular threading only if gevent import failed. Fixes #306.
Check for gevent presence in concert script
Test killing futures only on gevent enabled
Merge pull request #307 from ufo-kit/fix-306-async-module

Fix 306 async module
added PyqtgraphImageViewer