Fixed crashes random crashes in multithreading caused by useless creation of CheckUnusedFunctions instance per thread.
Refactorizations optimizing std::string usage:
1) Added global const std::string emptyString; object. It is used for:
- Empty string passed to a function, which takes the parameter by reference
-> Avoids constructor call (std::string::string(""))
=> Reduces binary size (25% for testrunner.exe, 5% for cppcheck-core.dll)
=> Avoids unnecessary calls
- Functions, that return a string by reference
-> Even functions that return an empty string in some branches can return by reference now.

2) Added overloads for TestFixture::assertEquals for the most common use cases:
-> Moves conversion from const char[] to std::string into a function, reducing code duplication in binary.
Updated solution to VS12
Visual Studio: Use testsuite.h as precompiled header (compiled into testrunner.cpp)
-> Reduces build time of project testrunner by 13 seconds (16%)
Refactorization: Moved condition in Token::tokAt() out of loop.
Benchmark code
Simplified check registration:
- Use sorted insert instead of calling std::list<Check*>::sort() on each insertion
- Removed DJGPP/__sun hack in check.h (should be obsolete by our compiler requirements for C++11