made gladus::buffer::unbind const

Either both bind and unbind are const or non-const. Still not decided on this yet.
added texture.hpp

The gladus::texture class provides a wrapper around an OpenGL texture of arbitrary target (GL_TEXTURE_2D, GL_TEXTURE_3D, etc.). Supported at the moment are the creation and destruction of a texture resource, as well as setting wrap and filter parameters. What's missing most importantly is a function to load data into the texture. Coming soon.