fixed the OpenGL includes

opengl.hpp now has the includes required to run properly on Linux.
added GLADUS_HAS_BINDING

The gladus/binding.hpp header now defines GLADUS_HAS_BINDING when used.
added error.hpp

The error.hpp header contains exception handling code. It defines a base gladus::error class which derives from std::exception. Based on this an opengl_error is defined that captures an error returned from glGetError(); and runtime_error which captures a nested chain of runtime_errors and one underlying opengl_error.

Additional functions are provided that help to check for OpenGL errors, or throw an exception if an OpenGL error is encountered. Two macros, if_opengl_error and incase_opengl_error, are provided to shorten OpenGL error handling code. The error checking functions may be converted into a nop via a preprocessor define.
added error handling to buffer.hpp

The gladus::buffer class now properly checks for OpenGL errors in the appropriate places, throwing gladus::runtime_error exceptions with detailed error messages where appropriate.
added state.hpp

The gladus::state class performs a reversible, exception-safe way of doing the equivalent of calling glEnable() and glDisable(). An instance of the class may be created, modified with enable(), disable() and set(), and the class will properly undo the changes upon destruction or reset.
now linking against OpenGL

The compilation.cpp test file is now properly linked against OpenGL upon compilation, to allow for syntactical and linker errors while developing the library. Fancy stuff.