Merge pull request #8 from ehenneken/master

updated requirements.txt
if no data is available, return 204 rather than 500
Merge pull request #9 from ehenneken/no_data

if no data is available, return 204 rather than 500
Removed unused imports
Merge pull request #10 from ehenneken/remove_unused_imports

Removed unused imports
blueprint update
Merge pull request #11 from ehenneken/blueprint_update

blueprint update
Forgot to import Blueprint from Flask
Merge pull request #12 from ehenneken/bugfix

Forgot to import Blueprint from Flask
added rate_limit to graphics views
Merge pull request #13 from ehenneken/rate_limit

added rate_limit to graphics views
update of README