implementation of algorithms: Dijkstra's, compare binary trees, invert binary tree, reverse linked list

[mzarnecki]

This pull request contains implementation of algorithms:

• Dijkstra's
• compare binary trees
• invert binary tree
• reverse linked list

and small refactoring for Bellman-Ford algorithm implementation

[mzarnecki]

@darwinz could you take a look at this PR when you have a moment?
I added some algorithms related binary trees, linked list and graph shortest path.
There is also second PR with neural network perceptron algorithm written in PHP.

[darwinz]

This is a great addition. Thanks for adding it!