Add downloading of coincidences as csv

Add a download form and streaming csv to download coincidences.
Each row in the csv contains an event, station number and coincidence
id.
Using the coincidence ids the events belonging to the same
coincidence can be found.
Add link to coincidence download form

Add link for to download form to the coincidences histogram pages
Fix some wrong links, improve url consistency

Fix wrong forms for coincidences and data downloading
Add slashes after urls for consistency
Add help text to coincidences form to explain N.
Merge pull request #59 from HiSPARC/stream_coincidences

Add streaming csv download for coincidences.
Similar to downloading of other ESD data,
but added coincidence id and station number columns.