Improve JpGraph setup procedure

[dregad]

JpGraph is no longer bundled with MantisBT.

As a result, the configuration option to specify the library's location (which was previously managed by the MantisGraph bundled plugin) is no longer available. It is therefore not possible to specify the location for the library, forcing users to store it in ./library.

Suggest to add a config option to allow user to set path to JpGraph to a location of their choice.

Alternatively, you may also want to consider a more modern graph library instead (e.g. jQuery.Gantt or similar), and bundle it with the plugin.