Support for accents, hard accents, tremolos and trills in guitar pro files.

This pull request contains support for accents, hard accents, tremolos and trills in GP4 and GP5 files (hard accents are supported in reading of GP4 files only). Tests are also present in the mtest/guitarpro directory which test all of the aforementioned features and successfully pass. Any questions/comments etc just let me know. Thanks!