pygbif v0.4.0

• changed base url to https for all requests; was already https for maps and downloads in previous versions
• occurrences, species, and registry modules gain docstrings with brief summary of each method
• pygbif gains ability to cache http requests. caching is off by default. See ?pygbif.caching for all the details (#52) (#56) via @nleguillarme
• made note in docs that if you are trying to get the same behavior as the GBIF website for name searching, species.name_backbone is likely what you want (#55) thanks @qgroom
• for parameters that expect a bool, convert them to lowercase strings internally before doing HTTP requests