in MacOS project GHUnit.h incorrectly imports GHTestUtils

fixes issue #105

to reproduce the problem:

1. clone the latest gh-unit
2. cd gh-unit/Project-MacOSX
3. build the MacOS framework: eg.  CC=;make clean;make
4. create a new Xcode project that uses the GHUnit.framework you just
created following these instructions:
http://gabriel.github.com/gh-unit/docs/appledoc_include/guide_install_macosx_4.html
5. in Xcode, build and run

expected: no problems
found:
/Users/moody/git/gh-unit-fork/Test/Test/GHUnit.framework/Headers/GHUnit.h:42:9:
'GHTestUtils.h' file not found

solution:
1. in GHUnit.h move GHTestUtils.h to #if TARGET_OS_IPHONE block
2. repeat step 3 and replace GHUnit.framework in Xcode project
3. build

it is possible that GHTestUtils _should_ be part of the MacOS framework but i did not see the two functions defined there:

void GHRunForInterval(CFTimeInterval interval);
void GHRunUntilTimeoutWhileBlock(CFTimeInterval timeout, BOOL(^whileBlock)());

referenced in the MacOS project