Adding GetPlainText methods, fixing charmap

[robarchibald]

Thanks for your awesome PDF library! It works great and is faster than some of the others I tested as well. Great work! I made a couple of changes that might be useful to you and others using this library.

I started from the ledongthuc fork and then added a fix to charmap encoding since it wasn't working. Added bfchar in addition to bfrange that was already implemented. Updated Font.Encoding() method so that it'll actually use the charmap encoding.

[rikvanmechelen]

Made the change that @martoche mentioned #21