Fix issue #10. Bitly usage is improved.

If the tool cannot connect to Bitly API, URL are not shortened.
Fix issue #6. It's now possible to specify time between two tweets in pelican_tweet_summary.py using configuration variable `SUMMARY_INTERVAL`.

Default value is 180seconds.
Add temp file to .gitignore
Fix a bug in Bitly usage. If Bitly configuration is set and Bitly API cannot be reached, Bitly API is not used.
FIx issue #8. Add possibility to set the number of days for which posts should be considered as new.
Add LICENSE file
Add History in header
Fix typo
Add ABOUT file
Add ABOUT file - Delete some headers in code