Fail to build a rate limiter if there is no backend.

Earlier versions of this assumed you could default fail over to a
simplebackend. Production showed this is not the case. There is no
reason for a failover and was a bad idea on my part. Now we croak if we
see there is not one given. Test suites have been updated accordingly.
Support JSON as the default serialisation for errors. Bug found by Lel on dev
Service printed line breaks between multiple sequences to STDOUT and not to the file handle
Adding tests for multiple sequences and text/plain.

Checking that the changes will not regress by adding two new tests. The
firsts makes a bad request for multiple sequences and makes sure we
correctly error. The second makes sure we have 1 sequence per line.
added assembly_info field for most endpoints
useful for making sure this is the correct assembly you are fetching from
REST version now 3.0.1 to include bugfix and additional assembly_name field in output
added assembly_info field for most endpoints
useful for making sure this is the correct assembly you are fetching from
Merge pull request #15 from Ensembl/hotfix/sequenceerrorbug

Hotfix/sequenceerrorbug
Merge pull request #14 from Ensembl/feature/ratelimit

Fail to build a rate limiter if there is no backend.