Add upper_bound for mined freq. subgraphs

[blumewas]

Hi, first of all thank you for the great work! As I mentioned in an earlier issue I used your package for the practical part in my Bachelor Thesis.

I have made some modifications in the program and want to share with other programmers around so that they can hopefully profit from my work.

Modification

As in the gSpan-paper described the experiments use an upper boundary for the max graphs generated. For some datasets I have used, the process seemed endless, especially with low suppport. So I have added the argument -mm <number> or --max_mining <number> to help the algorithm stop the mining process if the count of generated features reaches the <number> passed with the argument.

[blumewas]

pull request #19