ERT-600: Renamed docs/static -> docs/Static
ERT-600: Pure whitespace changes
ERT-600: Moved include of docs/CMakeLists.txt
ERT-600: Added an extra option ERT_DOC_INSTALL_PATH

If the optional variable ERT_DOC_INSTALL_PATH is set the documentation
will be installed in this location in addition to
${CMAKE_INSTALL_PREFIX}/documentation.
ERT-600: Setting path to images.

If ERT_SHARE_PATH is not set we try the relative location in
the source tree.
Merge pull request #577 from joakim-hove/ERT-600-fixup

Ert 600 fixup