Complete Java plugin system for functions and constants. Added standard library plugin project.
Merge branch 'jake/GUI' into jordan/plugins

Conflicts:
	MathsThing_GUI/src/uk/ac/uea/mathsthing/Functions.java
Fixed a problem that caused compiled versions of this to fail when loading plugins on Linux.
Added standard plugin libray to project. Added plugin security policy.
Fixed a problem with file paths on Windows and added plugin extensions for the GUI.
Created a better sample plugin for the extensions.
Fixed the PluginSystem throwing NullPointerException when no plugins were found.
Adds a rudy plugin for demoing of reading files in a plugin
Adds a security test plugin to show the security functionality blocking attempts at writing to files & closing the application.
Shows an error dialog when a plugin does something nasty when updating the formula
Minor changes to the product guide