Renamed three spannertools classes.

    spannertools.CrescendoSpanner
    spannertools.DecrescendoSpanner
    spannertools.HairpinSpanner

    ==>

    spannertools.Crescendo
    spannertools.Decrescendo
    spannertools.Hairpin

MILESTONE: All top-level classes finally have musically relevant names:

    Articulation
    Beam
    Chord
    Clef
    Container
    Crescendo
    Decrescendo
    Duration
    Dynamic
    Fraction
    Hairpin
    Measure
    Multiplier
    NamedPitch
    Note
    Offset
    Rest
    Score
    Slur
    Staff
    Tempo
    TimeSignature
    Tuplet
    Voice

Some classes in spannertools remain to be renamed. We will follow the names
provided by LilyPond's list of spanner grobs when possible
(http://lilypond.org/doc/v2.17/Documentation/internals/spanner_002dinterface),
and use the suffix "Spanner" for those artificial constructs unique to Abjad,
such as the StaffLinesSpanner.