Release 0.3.4

1) Fixed the browser timeout issue.
2) Added Cancel button that halts the execution of ClaferMoo and all its dependencies.
3) Fixed an issue with saving instances (it did not work in the last release).
4) Errors are more verbose (some exceptions are recognized, and the appropriate error is displayed).
5) Improved server-side and client-side timeouts.
6) The Visualizer does not crash when ClaferMoo crashes.
7) Introduced sessions like in Clafer Configurator.
8) Fixed an issue with removal of temporary files and directories (a directory may be requested to be removed earlier than files in it).