[Extensions] Make RegisterExternalExtensionsInDirectory return a list of extensions name

Having XWalkExtensionServer::RegisterExternalExtensionsInDirectory returning
a vector of the loaded extensions name has proven to be useful for the XWalk
Extensions Shell. It is harmless for callers who doesn't need that information.
[Extensions] Add XESh: XWalk Extensions Shell

This commit adds a tool called XESh - a very simple Javascript
shell that loads external extensions for testing.

Usage: ./out/Release/xesh --external-extensions-path=$CODE/tizen-extensions-crosswalk/out/Default

After that you will get a prompt 'xesh>'. Just type any
javascript command or test your extensions.

What will be missing after this initial patch:
- centralize all shell 'handling' to a singleton and replace
fgets and fprintf calls to function calls of this class;

- make this shell handler use GNU readline so we get a better
shell for free :);

- Register the 'internal' native module;

- Make XESh rocks!
(a.k.a.: Any other shell cosmetics one feel like doing.)
[Extensions] Add a (simple) manual test for XESh

This manual test runs XESh passing an input file that
uses the echo extension. If the extension gets loaded then
it means the javascript command got executed and XESh has loaded
the echo extension properly.