add a screenshot
compile as C too

With these changes, the project and header files
can be compiled as C as well, tested with gcc 4.8
and clang 3.3. I also tested with clang++ again to
see if it still cleanly compiles as C++ codem which
it does.
Merge pull request #3 from Aktau/compile-as-c-fixes

compile as C too
Merge pull request #2 from Aktau/add-screenshot

add a screenshot
Changed compiler to C
Update screenshot

- update screenshot for a "pixel perfect" version
Moved implementation outside include guards, added font count as param

- moved *_implementation define outside include guards as per Sean's
recommedation
- added font count as parameter for fontstash_create
fix permissions on files

Source files and .ttf files need not be executable
cache the glyph id and use it for kerning

When using stbtt_GetCodepointKernAvance, sbt_truetype
has to convert both codepoints internally to glyph ids
with sbtt_FindGlyphIndex, which is a rather heavyweight
function that can be entirely avoided by caching the
glyph and calling stbtt_GetGlyphKernAdvance.
Updated readme, screenshot and remove unnecessary header files from demo
Merge branch 'cache-glyph-id-and-fix-make' of https://github.com/Aktau/fontstash into Aktau-cache-glyph-id-and-fix-make