install_requires=['requests>=1.0.0, <2.0.0'] implies that request is
uninstalled if a more recent version is installed. This behaviour
will be removed, however module seems to work even with more
recent version of request

Scaling n of request per seconds if more processes are active in a
limited account

Minor fix

if a request fails with a known transitory error (something bad has happened,
Something went wrong while fetching from LDFeatureContainerAdaptor), it could
be done automatically (for a certain amount of retries) before raising exceptions

By settings timeouts and internal retries I can improve performances
(even with sleeping times between each retry)

The main problem when submitting more queries than permitted is that
ensembl limit 15 request per second, so there's no need to fix the
internal number. By submitting more than 15 request per second, the
EnsemblRestRateLimitError exception will be reaised. The user will check
his code to deal with exceptions or avoding to submit multiple clients
using pyEnsemblRest API

An ensembl id used in test was removed recently. I've splitted code
in order to do small functions in my classes