Added get_instance to avoid multiple instances of GUMP

[MayurVirkar]

This is a fix for two issues in GUMP
1: When you use is_valid() and filter_input(), $gump = new self() creates multiple unwanted instances of GUMP.
2: If you extend GUMP class and if the extended class has a constructor defined, issue mentioned in point 1 tends to call constructor multiple times hence messing up the code logic.

Fix contains singleton pattern implementation for GUMP.
This way, instance is created once and passed around multiple times also resulting into performance improvement and less resource usage.

Have tested the changes against all files in example directory and also currently using it in one of my live projects.

[sn]

great idea and thanks for the fix! I've been wanting to do this one for a while now.

[MayurVirkar]

Welcome :)
GUMP is a nice and handy library.
Will try to contribute more in future :)