Implement repository crawling system. Close #4

GitHubCrawler crawls GitHub according to CSV file which has repository information.
#### Done
* Repository clone
* Fetch pull requests
* Fetch commit IDs corresponding to pull request which has one commit at least.

#### Doing
* Taking diff (I will make another issue.)
http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap03.html
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