Specify dir. layout and stylesheets in common templates

These options should be common to all OPM modules to get the same look
for the generated documentation.
Only add option for shared libraries once

The way the test was done previously, it was added for *every* option
after `--enable-shared`, not only that one.
Allow to search for multiple libraries in package

Some packages are split up in several library files, where each of them
doesn't have their own CMake find-module. So we cannot put this list in
the prerequisites. Unlike the headers, where we just add the directory,
all the libraries must be explicitly named on the link line. Thus, we
should allow to specify more than one name in the lib parameter.

The first name specified is designated as the "primary" library and is
assigned to the _LIBRARY variable; the others occur in _LIBRARIES as if
they were prerequisites.

Note that the build system cannot replicate this setup itself; it always
assumes that we are building (at most) one library from the source
files.
Added CMake find modules opm-parser and cjson
Added opm-parser-prereqs.cmake
Updated modules for Findcjson and Findopm-parser
Updated modules to find Opm-parser
Merge pull request #64 from bska/064_cmake

Synchronise build system with opm-core