Added a method to return png as file.
Added block manipulation for png tempfile.
Refactored a bit for clarity.
Removed useless naming.
Removed inconsistent behaviour of as_png_file by making block required.
Merge branch 'feature/png_as_file' of github.com:quipper/capturemath into feature/png_as_file

Conflicts:
	lib/capturemath.rb
	spec/capturemath_spec.rb