Implement .rpm package gulp task

.rpm packages can now be build with:

    gulp vscode-linux-<arch>-build-rpm

The resulting package will be output to ~/rpmbuild/RPMS/<arch>/ and can be
installed on a Red Hat-based distro with:

    yum localinstall <path>

Note that Debian systems can build the package after installing the 'rpm'
package:

    apt-get install rpm

This was tested building 64-bit on Ubuntu 15.10 and installing on CentOS 7.

Fixes #3595

resources/linux/code.desktop

@@ -0,0 +1,10 @@
+[Desktop Entry]
+Name=@@NAME_LONG@@
+Comment=Code Editing. Redefined.
+GenericName=Text Editor
+Exec=/usr/bin/@@NAME@@ %U
+Icon=@@NAME@@
+Type=Application
+StartupNotify=true
+Categories=Utility;TextEditor;Development;IDE;
+MimeType=text/plain;