Adding support to persist views each frame.

* Added "dump_image_file_prefix" attribute to channels
* The corresponding evn variable is EQ_CHANNEL_SATTR_DUMP_IMAGE_FILE_PREFIX
* Added default implementation of FrameWriter that writes to a file
* Some refactorings in order to make the code a bit more readable
Setting name of parameter to dump image to file:

* EQ_CHANNEL_SATTR_DUMP_IMAGE
* dump_image
Merge branch 'persistframes' of https://github.com/julitopower/Equalizer into julio
CR #254
Adress open TODO on #254
Fix build
Update doc (#254)
Fix build w/o optional dependencies
Merge pull request #256 from tribal-tec/master

Fix build w/o optional dependencies
Merge branch 'master' of https://github.com/Eyescale/Equalizer

Conflicts:
	eq/client/glx/messagePump.cpp
Merge remote-tracking branch 'origin/julio'
Fix Wunused-param
Improve WGL pipe size detection code