[bug]Fixed bugs when rendering dot file into graphics which is introduced by tree.export_graphviz

When exporting a tree into a dot file using `tree.export_graphviz`,  the dot file is usually like this 
```
    digraph Tree {
        0 [label="A <= 1.5000\nerror = 0.451898202175\nsamples = 19396\nvalue = [  6690.  12706.]", shape="box"] ;
        1 [label="B <= 1.5000\nerror = 0.484204229212\nsamples = 6690\nvalue = [ 2667.  4023.]", shape="box"] ;
        0 -> 1 ;
        2 [label="C <= 1.5000\nerror = 0.429043311043\nsamples = 12706\nvalue = [ 4023.  8683.]", shape="box"] ;
        0 -> 2 ;
    }
```

However when sample number is large, `tree.export_graphviz` will create a dot file like this
```
    digraph Tree {
        0 [label="A <= 1.5000\nerror = 0.451898202175\nsamples = 2.48325 e6\nvalue = [  2.470544 e6   1.2706 e4 ]",
        1 [label="B <= 1.5000\nerror = 0.484204229212\nsamples = 2.470544 e6\nvalue = [ 2.667e3  2.467877e6]", shape="box"] ;
        0 -> 1 ;
        2 [label="C <= 1.5000\nerror = 0.429043311043\nsamples = 1.2706 e4\nvalue = [ 4.023 e3  8.683  e3]", shape="box"] ;
        0 -> 2 ;
    }
```

The scientific notation(like `value = [  2.470544 e6   1.2706 e4 ]`)  will broke the dot file rendering process like follows[1][2]:

```
    dot -Tpdf file.dot -o file.pdf
```

The patch will transform the scientific notation(like `value = [  2.470544 e6   1.2706 e4 ]`) into normal one(like  `value = [  2470544.   12706]`).


[1] http://scikit-learn.org/stable/modules/tree.html#classification
[2] http://scikit-learn.org/stable/modules/generated/sklearn.tree.export_graphviz.html#sklearn.tree.export_graphviz