Add string::c_str() to several functions.

There were a number of places where a std::string was being used in a function calls that expected a c string.  These instances were trivally fixed by adding a call to string::c_str().

Note, I'm not sure if this is a GCC specific issue, however even if it is a case of MSVC simply letting these usages slide it can't hurt to be explicit.
Merge pull request #7 from DocHoncho/gcc-fixes

GCC fixes: explicit conversion of std::basic_string to const char*
Add missing typenames in ImageImpl
Fix get_app_path() impl for Linux and Mac OS X
Add missing inline modifier for ImageImpl<>::clear() specializations
Mac OS X fix: ALLEGRO_HAVE_STDINT_H is already defined in alplatf.h
Add some German and Russian Unicode characters to font.png (to fix issue 294)
Fix new Unicode font, issue 294 (missing range of Cyrillic script)