gsdx-ogl: add preliminary support of external shader fx
gsdx-ogl: add a gui option (linux) to select external shader

Note: of course it requires a glsl shader ;)

On windows, you can set the path on the ini file. Here an example with linux path:
shaderfx_conf = /home/gregory/playstation/emulateur/pcsx2_merge/bin/GSdx_FX_Settings.ini
shaderfx_glsl = /home/gregory/playstation/emulateur/pcsx2_merge/bin/shader.fx
gsdx-ogl: support latest fxaa version

Only tested on Nvidia, please report any issue with your driver

Note: requires GL4 GPU
gsdx-ogl: port Asmodean shader to GLSL

I only checked the default configuration (on Nvidia). Please report any issue with it.
Merge pull request #343 from PCSX2/shader-loading

GSdx ogl: external shader loading + FXAA