markup-commands rest-by-number and rest (issue 6850073)

[thomasmorley65]

Reviewers: ,

Message:
Please review

Description:
markup-commands rest-by-number and rest

Introduces two new markup-commands:
rest-by-number and rest
similiar to the existing note-by-number and note.
Two regression-tests for them are added.

Please review this at http://codereview.appspot.com/6850073/

Affected files:
  A input/regression/markup-rest-styles.ly
  A input/regression/markup-rest.ly
  M scm/define-markup-commands.scm