add parse layer link section

add layer link section; layer link represented by array in psd;
for example: 
1.
![](http://ww3.sinaimg.cn/large/6d4195cfgw1emanc9f0p0j206n03nwef.jpg)

layer3 and layer1 are linked;

then the array is [1, 1, 0]

2 if there are more layer links

![](http://ww4.sinaimg.cn/large/6d4195cfgw1emangjopmgj206l05p74b.jpg)

layer3 and layer1 are linked; layer4 and layer2 are linked;

then the array is [1, 2, 0, 1, 2]

3 if there are some group layer ,but not linked;

![](http://ww1.sinaimg.cn/large/6d4195cfgw1emank0x7drj206s06gq31.jpg)

layer3 and layer1 are linked; layer4 and layer2 are linked;

then the array is [ 0, 1, 2, 0, 0, 1, 2 ];

yes, we find group layer create 2 element in the array; here, I call them groupStart and groupEnd;

4 if there are some group layer, and they are linked;

![](http://ww4.sinaimg.cn/large/6d4195cfgw1emano1akwoj206h06aaa5.jpg)

layer3 and layer1 are linked; layer4 and layer2 are linked;  folder1 and layer5 are linked;

then the array is [ 3, 1, 2, 0, 3, 1, 2 ]; we find the groupStart is not zero, and the groupEnd also is zero;

Through the above,find:
 same linked layers , they have same number, not zero;
group layer has two numbers, and the groupEnd always zero;

OK, through some processing we could get which layer are linked mutually