Implemented LilyPondFile.__getitem__().

NEW: should be included in the 2.15 release notes.

The method accepts a name string.

Example:

    >>> lilypond_file = lilypondfiletools.make_basic_lilypond_file(note)
    >>> lilypond_file
    <LilyPondFile(4)>

    >>> lilypond_file['header']
    <Block(name='header')>

    >>> lilypond_file['layout']
    <Block(name='layout')>

    >>> lilypond_file['paper']
    <Block(name='paper')>

    >>> lilypond_file['score']
    <Block(name='score')>

This makes it considerably easier to retrieve the various elements in a
LilyPondFile.

The method raises a key error when no item with name is found in LilyPondFile.