Changed the hard coded path used in the qtjsruntime tests

The hard coded path does not work on Windows as MSBuild creates the target
output in a folder dependant on the build configuration.

Instead of using a hard coded path we can use the target name here as the
add_executable command will create the executable based on the platform
specific location

http://www.cmake.org/cmake/help/v2.8.12/cmake.html#command:add_executable
Fixed the NodeJS binary not being copied on Windows

The custom command to copy the NodeJS executable was never being executed
causing the include of the jsfileslist.txt to fail later on as it was never
being created.

I have changed this to an execute process so it is copied after the
download is complete.
Moved the copying of node_modules

Removed a duplicate step to copy the node_modules and moved the existing
step to copy this directory to execute for all platforms
Added some instructions for building webodf on Windows
Merge pull request #407 from satsuper/windows-build-fix

Further fixes for building on Windows
Fix loading of webodf.css in qtjsruntimetest.
If the <link/> element does not load the css, load the webodf.css in a <script/> element.
Also avoid loading webodf.css more than once.
Use reference counting to make sure that the webodf.css is not removed prematurely.
Improve dependencies for the tests.
NodeJS, qtjsruntime and manifest.json-target are top-level targets. Dependencies between top-level targets should be added with add_dependencies().
Always use <script/> to load css.
Using <link/> can cause problems in certain browsers, notably those based on
QtWebKit. By using only <script/> the behavior varies less across different
ways in which WebODF is used.