Fix clean-all target and robust link creation.

Move creation of links from source tree to build tree to CMake lists.
Add CMake target clean-links.
Automatically collect all links and delte tehm by this target.
Fix wrong directory name for GTest build subdirectory.
Clean unnecessary targets in main makefile.
Small changes in README to follow current Petsc
Merge branch 'master' of github.com:flow123d/flow123d
Small changes in README.md
Can set root build directory by BUILD_SPACE var.

Necessary for Jenkins builds.
Merge branch 'master' of github.com:flow123d/flow123d
Add include necessary for new boost.

... in particular boost/program_options
Changes to satisfy g++ 4.4
Added embeded icon with build status to display at GitHub
Merge branch 'master' of github.com:flow123d/flow123d
Small changes in README.md
Consistent headers in README.md
fix - missing cmath library
Merge branch 'master' into PE-trunk-interpolation.
(Build process was improved, fixed clean-all in master)

Conflicts:
	unit_tests/CMakeLists.txt
Directives for constants, make only P1 interpolation.