V1 - Export Files from Dev-App

This PR is part of implementation of task #47 .

Files can now be exported from desired Dev-App working repository.
HTTP GET request sent to 'api/file/download' with query string as the path of file to be downloaded will return a 'Save As' dialog in the browser.
Only a single file at a time can be downloaded. No support for direct folder download.
File inclusion vulnerability is put in check so that files outside Dev-App's repos folder (such as /etc/passwd ) cannot be accessed.
SetTimeout() is used for making this work in Firefox.

solettaproject / soletta-dev-app