Build doc with cmake, unify target names in makefile.

- use cmake to build LaTeX user manual
- added UseLATEX.cmake which supports LaTeX builds
- unified target names in makefile
- added target for documentation build with input reference_manual
Delete empty files in documentation.

- there where empty files not used in reference_manual
Fix error in documentation.

- there were included files which were deleted in last commit
- cmakelist in documentation commented
Makefile improved, doc fixed.

- targets renamed
- help target created
- documentation fixed - figure links
Merge pull request #67 from flow123d/PE-cmake-doc-build

Improved documentation build system and makefile.

- target names changed and unified
- build reference manual with cmake in build_tree
- reference manual directory cleaned, images sorted in separate dirs