Rename misnomer POP_CHAR to PEEK_CHAR.
While parsing token data, we use the POP_CHAR macro to 'peek' at
character data. This behaviour is noted in the comments for the macro,
yet the definition is left as 'pop'. Changing to PEEK_CHAR does not
imply that the character being observed is removed.
Fix broken build by using ADVANCE_CHAR macro return.

We forget to check or use the return value of the ADVANCE_CHAR macro,
and upon compilation an error is thrown because of its lack of use. This
patch checks to see if the macro was successful, and if not replaces the
offending character with a replacement.
Merge pull request #71 from WillDignazio/master

Fix Broken Build, Check ADVANCE_CHAR