Minor changes to make sigma publishable to NPM

Fixing #186. Here are some more details about the commit itself:
 - Updated conrad.js
 - Added a dirty script to fix exports: sigma.export.js
 - Fixed last references to window
 - Added the npmPrePublish Grunt task, and linked it to `npm prepublish`
 - Added .npmignore rules and fixed some in .gitignore
 - Updated package.json, changing the name from "sigma-js" to "sigma"
Added try/catch for control WebGL options in "Hard Environments"
sigma.utils.zoomTo() almost done. Only left the inital checking of the animation settings and the documentation of the function.
sigma.utils.zoomTo() done! Also the three events when it is possible to use it are using it now.
Merge branch 'master' into svg-renderer
Merge branch 'svg-renderer' of github.com:jacomyal/sigma.js into svg-renderer
Fixed #241: Improved canvas renderer

The canvas renderer no more uses two different canvases to draw edges and nodes when the `'batchEdgesDrawing'` setting is truthy:
It uses `context.globalCompositeOperation = 'destination-over'` instead.
Fixed #244: (min|max)(Node|Edge)Size as strings

The settings minNodeSize, maxNodeSize, minEdgeSize and maxEdgeSize now work both as number or string.
Merge branch 'svg-renderer' of github.com:jacomyal/sigma.js into svg-renderer
Merge branch 'master' into svg-renderer