import pybedtools only when it's used

it does something on initialization that takes ~2.5s, which makes
bcbio_vm.py noticably slow
Merge pull request #660 from jwm/import_pybedtools_on_demand

Import pybedtools only when it's used
Cleanly skip snpEff annotation if not defined in input file for other organisms. Thanks to George Marnellos
Enable automated download of pre-prepared data indexes. Allows bootstrapping runs on minimal AWS machines
Explicitly require click installation to avoid errors from chanjo. Fixes #661