Add FindPatch.cmake

FindPatch.cmake is used on Windows (MSVC) platforms to find patch.exe.
It currently only looks in msysgit/bin (and system paths) but this can
be easily extended.

PATCH_SCRIPT_PATH is now used to specify the location of patch.sh/bat.
As such SCRIPT_PREFIX has been removed.

Rationale for change:
* Modifying your path may require admin permissions for MSVC
* CMake now exits with an error message on MSVC if patch.exe cannot be
found
* Fewer manual steps when setting up Desurium on MSVC
Remove reference to TortoiseGit

TortoiseGit does not include patch.exe as indicated (msysGit does
however) and as such is superfluous.
Correct reference to Patch.bat

I missed a reference to patch.bat in commit
6d5aeb3607460ecf3a41947a0621862cb654c1de.
Substitute @...@ with ${...} in Patch.bat

This should improve readibility.
Check for patch on all systems

Ensure patch is available (and exit with an error if it can't be found).
Move patch.bat into CMAKE_GEN_SRC_DIR/scripts
Merge pull request #475 from harmonise/FindPatch

add FindPatch.cmake