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)
Merge branch 'ui-dev' of https://github.com/DocHoncho/aseprite into ui-dev