create new namespace 'data-sink'

Contains the console output method now. Can later contain db persistence.
The lein console command takes now the desired max number of concurrent requests as a third argument; currently all we do is print it out on the console though.
Methods for concurrent crawling added; untested though; depth monitoring is still missing
All methods for concurrent crawling added but not currently active and tested.
delete old synchronous crawler code
'concurrent crawling' works, but only for the first url, adding links to the queue does not work yet.

- resolved a circular reference problem between functions in the core
- adding meta-data to a string didn't work, back to objects on the queue
- add a try and catch block in the main function
- Clojure version updated to 1.6.0
bug that kept blocking the send to the urls-to-be-crawled queue removed
crawler works again, but two problems:

1. no real concurrency
2. stops/blocks on a link to a .pdf file
small fix: pdfs are not crawled for now
fixed the not-a-real-concurrency issue; now blocking at a certain point
reduce console output