New feature: Add Detail Descriptions of Problems

Regarding to this issue: https://github.com/mjnaderi/Sharif-Judge/issues/4, I have eventually done due to the wonderful X'mas holiday :)

The file structure of the zip file contains only test cases before. Now, for each problems, there can be 'desc.md' or 'desc.html' for markdown and html formatted description respectively. Any desc.md will be change to desc.html just after uploading the zip file. If both exist, the converted desc.md will override desc.html.

Extending from http://docs.sharifjudge.ir/tests_structure, the file structure is now:
```
.
├── p1
│   ├── in
│   │   ├── input1.txt
│   │   ├── input2.txt
│   │   ├── input3.txt
│   │   ├── input4.txt
│   │   ├── input5.txt
│   │   ├── input6.txt
│   │   ├── input7.txt
│   │   ├── input8.txt
│   │   ├── input9.txt
│   │   └── input10.txt
│   ├── out
│   │   └── output1.txt
│   └── tester.cpp
│   └── desc.md / desc.html / both (though desc.html will become useless) / not provided
└── p2
    ├── in
    │   ├── input1.txt
    │   ├── input2.txt
    │   ├── input3.txt
    │   ├── input4.txt
    │   ├── input5.txt
    │   ├── input6.txt
    │   ├── input7.txt
    │   ├── input8.txt
    │   ├── input9.txt
    │   └── input10.txt
    └── out
        ├── output1.txt
        ├── output2.txt
        ├── output3.txt
        ├── output4.txt
        ├── output5.txt
        ├── output6.txt
        ├── output7.txt
        ├── output8.txt
        ├── output9.txt
        └── output10.txt
```