webmenu: add python to build dependencies

node-gyp rebuild requires python:

gyp ERR! configure error
gyp ERR! stack Error: Can't find Python executable "python", you can set the PYTHON env variable.
gyp ERR! stack     at failNoPython (/usr/lib/node_modules/npm/node_modules/node-gyp/lib/configure.js:109:14)
gyp ERR! stack     at /usr/lib/node_modules/npm/node_modules/node-gyp/lib/configure.js:56:11
gyp ERR! stack     at F (/usr/lib/node_modules/npm/node_modules/which/which.js:43:25)
gyp ERR! stack     at E (/usr/lib/node_modules/npm/node_modules/which/which.js:46:29)
gyp ERR! stack     at /usr/lib/node_modules/npm/node_modules/which/which.js:57:16
gyp ERR! stack     at Object.oncomplete (fs.js:297:15)
gyp ERR! System Linux 3.2.0-32-virtual
gyp ERR! command "node" "/usr/lib/node_modules/npm/node_modules/node-gyp/bin/node-gyp.js" "rebuild"
gyp ERR! cwd /tmp/buildd/webmenu-0.3.4/node_modules/posix
gyp ERR! node -v v0.8.15
gyp ERR! node-gyp -v v0.7.1
gyp ERR! not ok
npm ERR! posix@0.0.10 install: `node-gyp rebuild`
npm ERR! `sh "-c" "node-gyp rebuild"` failed with 1
npm ERR!
npm ERR! Failed at the posix@0.0.10 install script.
npm ERR! This is most likely a problem with the posix package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     node-gyp rebuild
npm ERR! You can get their info via:
npm ERR!     npm owner ls posix
npm ERR! There is likely additional logging output above.
webmenu: release 0.3.4