Added three new stringtools functions.

    * stringtools.to_dash_case()
    * stringtools.to_snake_case()
    * stringtools.to_upper_camel_case()

Examples:

    >>> stringtools.to_dash_case('scale degrees 4 and 5')
    'scale-degrees-4-and-5'

    >>> stringtools.to_snake_case('scale degrees 4 and 5')
    'scale_degrees_4_and_5'

    >>> stringtools.to_upper_camel_case('scale degrees 4 and 5')
    'ScaleDegrees4And5'