Add Sliceable Switch REST API independent from Apache httpd

This commit adds the sliceable switch REST API implementation using PSGI/Plack. It is independent from Apache httpd.

The sample configuration of Sliceable Switch REST API cannot be used with Apache 2.4. It only works with Apache 2.2 or. Ubuntu 14.04 is shipped with Apache 2.4 and this commit makes the sliceable switch work on Ubuntu 14.04.

Note that PSGI is a web application interface in Perl which decouples web server implementations and web application  frameworks. The new restapi.psgi application works with Apache httpd too with a small apache configuration.

Fixes #36