Added preliminary version of Projection Volume Style.
Corrected the first local maximun and minimun condition for the maximun and minimun projection styles
Make some changes and modified the currently implemented styles accordingly
  * Use of inheritance to not repeat the common shader text in all the styles
  * Modified the volume rendering integral (the way the color is accumalated)
  * Added a global light equation without considering materials
  * Added a function to get normals from texture when provided or calculated from central differences if not.
Modified the OpacityTest to fit the new way to accumulate color and added a new test case for the projection volume style
Merge pull request #239 from aarbelaiz/volume_rendering_styles

Volume rendering styles