Add a linkchecker for Jekyll Site

[souravsingh]

This PR is related to Issue #62 for checking broken links.

[peterjc]

Good news - it works :)

Bad news - lots of broken links :(

Unless this is setup as a TravisCI build target which is allowed to fail, it will not be useful to commit this yet. Hopefully we can just fix the links...

You should probably tell linkchecker to only look at ./_site as per the example on #62 which would avoid the false warning from ./_layouts/default.html etc.

I just resolved the Gmane problem with https://github.com/biopython/biopython.github.io/commit/bd236b75a4b97bd0d34fd7787af807934f2ddc3c and a malformed link this found with https://github.com/biopython/biopython.github.io/commit/97005af1795ca26cea42e4ced38b412602771021

Most of the errors are about the GitHub editing links, can you see what is going on there?

[peterjc]

Also as per the example on #62, using something like --only-4xx might be needed for all the (mostly valid) DOI links are getting Connection refused, and GitHub links returning HTTPTooManyRequests

[souravsingh]

@peterjc I will look into the issue with the false alarms.