API: Added M_ReadFileIntoString()

A convenient method of reading a file or WAD lump into a string.
Map|World: Cleanup
libdeng1|API: Added Str_StartsWith()
libcommon: Derived HexLex (class) from Hexen's script parser

A (common) lexical analyzer for Hexen definition/script syntaxes.

Todo: Cleanup
Refactor|Hexen: Use HexLex to parse MAPINFO data
Refactor|Hexen: Use HexLex to parse ANIMDEFS data
Refactor|Hexen: Use HexLex to parse SNDINFO/SNDSEQ data
Merge branch 'master' of github.com:skyjake/Doomsday-Engine