Fix exports (remove export_as hack)

This PR removes the support of `export_as` from the `generate-exports.py` script, and our abuse of this argument in `.exports` files (see this [thread](https://github.com/openlayers/ol3/pull/930#issuecomment-30466292)).

With this change a `goog.require` will always be generated when `@exportSymbol` is used. If the exported symbol's last component starts with a capital letter (e.g. `ol.source.OSM`, `ol.webgl.SUPPORTED`) then a `goog.require` for the full name is generated (e.g. `goog.provide('ol.source.OSM')`, `goog.provide('ol.webgl.SUPPORTED')`). If the exported symbol's last component starts with a small letter (e.g. `ol.animation.bounce`) then a `goog.require` excluding the last component is generated (e.g. `goog.require('ol.animation')`). This is consistent with https://github.com/openlayers/ol3/commit/8ca3f13bd2956a8cafa78c15ec2c27dc45080ac1.

Please review.