A lot of things going on in this commit:

- added tangents computation for geometries
    - mesh must have UV coordinates
    - to be called explicitly once geometry is loaded like this: geometry.computeTangents()
    - tangents are stored in Vertex objects
    - quads are not solved properly (though workaround hack seems to work at least somehow, as far as each vertex appears at least somewhere)

- extended VBOs in WebGLRenderer to include tangent streams (when available)

- added "normal" shader to ShaderUtils (to be used with MeshShaderMaterial)
   - Blinn-Phong with one directional and one point light (7 varyings gone, just one spare)
   - normal maps are in tangent space
   - displacement maps use simple luminance value (could be changed if better precision is needed)

- displacement mapping uses vertex texture fetch
    - this requires GPU with Shader Model 3.0
    - not currently supported in ANGLE
    - for this, added "supportsVertexTextures" method to WebGLRenderer API, so that application can handle this