Renamed StyleMap and Style to OptionMap and Opts respectively

To allow plot parameters to be configured via the same mechanism as
styles, StyleMap and Styles have been renamed to OptionMap and Opts. The
new OptionGroup component allows easy definition of multiple categories
of options with a shared set of keys. The 'options' object is the
default instance of this class that is a handle allowing plotting and
style options to be set. The file has been renamed accordingly.
Added style_opts parameter to Plots to specify valid matplotlib options
Minor simplification to how AnnotationPlot filters style keywords
Removed PlotOptsMagic and replaced StyleMagic with more general Optsmagic

The IPython extension has been updated to support the new options
system, allowing the functionality of PlotOptsMagic and StyleMagic to be
unified under OptsMagic. The %opts and %%opts line and cell magics can
simultaneously configure plotting options and matplotlib style options
on all View objects. These magics feature tab-completion and helpful
warning messages when invalid keywords are used.
Fixed z-ordering in SheetPlot and variable clash in AnnotationPlot
Updated OptionMap to handle objects without style or with style lists
Fixed collect method to correctly iterate over Overlays in Stacks