Create meta extraction algorithm

[Godley]

Algorithm will work as follows:

• upon refresh or creation, check the given folder for files
  • for MXL files, check if there is an unzipped copy with the same name in the folder. if not, unzip it. issue create file unzipper #184
  • for XML files:
    • get a list of all files in the database ( compare list of files in DB to list of files in folder #185 )
    • for any files not in the list, ( compare list of files in DB to list of files in folder #185 ) parse the file ( create an xml file parser #186 ) and sort the data into a dict containing:
      • filename
      • title
      • composer
      • instruments, which should be organised into a list of dicts containing:
        • name
        • transposition, which should again be a dict containing
          • octave change
          • diatonic change
          • chromatic change (if any of these are unchanged, set them to 0)
      • keys, which should be a dict with instrument names as keys, and a list of dicts containing fifths and mode as indexes
      • clefs which should be the same as keys
      • time which should be a list of dicts, with "beat" and "b_type" as indexes
      • tempos which should be a list of dicts with beat and either minute or beat_2 as indexes. Beat and beat_2 should be the rhythmic name in either English or American terminology. Where beats are semi/hemi/demi they should not contain spaces, and where beats are dotted this should be denoted by dots put at the end of the beat name.
    • for any files in the list which aren't in the folder, delete them or archive them.

This will all be managed by the musicManager class described in issue #189