show plot when query parameter is given

- for example, query parameters look like following
  `plot_type=line&x_axis=net_size&y_axis=real_time#!tab-plot`
return irrelevant keys in ParmeterSetsController#_line_plot
ParameterSetsController#_scatter_plot returns irrelevant keys
refactoring: removed unused parameter
refactoring: removed unused variables
ParameterSetsController#_figure_viewer returns irrelevant keys
trivial refactoring
show URL of the line_plot
trivial refactoring
show url of scatter plot
show url in figure_viewer
bug fix in gnuplot script rendering

- when real_time or cpu_time is specified as y-axis, it causes an
  exception
display plot actions in dropdown menus
make plot_url on server side
select plot_url when clicked
trivial refactoring
Merge pull request #282 from crest-cassia/url_for_plot

URL for plot