first crack at setting up class-based parsers
updated CONTRIBUTING guide for class-based parsers
converted the rest of the parsers to subclassing textract.parsers.utils.BaseParser
fixed pptx parser
unicode sandwich! (for anything that uses a ShellParser)
unicode sandwich for all!
not using the md5 function
standardized documentation on the Parser classes
updated documentation with a few things in anticipation of merging in the class-based parsers
got everything merged in from master, including xls/xlsx parsers and all bug fixes
ignoring incompatible code points when re-encoding to byte-strings
merging in #39