MAINT: changed default phidp_field in calculate_attenuation.

The pyart.correct.calculate_attenuation function will now use the
`corrected_differential_phase` or `unfolded_differential_phase` fields when
present in the radar object when the phidp_field is not specified in the
function call.  When neither of these fields are available the
differential_phase field will be used.

Closes #109.
ENH: plot method added to RadarDisplay

The plot function of the RadarDisplay calls the correct plot_*
function to create a plot appropiate for the given scan_type.
MAINT: renamed tilt parameter of plot_ to sweep

tilt parameter of plot_* methods in RadarDisplay and related plotting
objects was renamed sweep.
ENH: radar_plot command now supports VPT scans
MAINT: cylp default LP method

Made cylp the default LP method in phase_proc_lp.  Removed unnecessary print
statements from phase processing functions.
EXAMPLES: updated correct examples
EXAMPLES: updated mapping examples
EXAMPLES: updated plotting examples