Add some node logic [issue_1]

A project' infrastructure was build.
Travis ci integration was implemented.
Some node logic was added.
Edit .travis.yml to trigger a build process

One string was modified to begin a build process
Implement only node adding in a graph structure

Implement the constructor in graph class
Implement only adding in a graph structure.
Realise a depth-fisrt search method

Depth-first search method was realised.
When it finds the way, the way will be shown as a string
(A->B->C->D)
When no nodes are found, then a NodeNotFoundException will be generated
Implement breadth first search algorithm

Breadth first search algorithm was implemented.
Also Depth first search alogorithm was modified.
Detele GraphException_test file

GraphExcpetion test file was deleted.
It is nothing to test in this file.