Biothings-client

Update HTTP request calling signature to match server update. (17)

Support additional list-typed query parameters. (12)

Fix a bug when certain query parameters are passed in as lists. (12)

Use doc_type key in metadata to generate client without having to specific biothings_type (6).
Have a more consistent user-agent pattern (7).
Fixed the issue when passed query terms contains white spaces in querymany method (8).
Added warning when as_dataframe set to True and fetch_all is True (9).

Added client-specific doc-strings to match with current mygene.py and myvariant.py clients.
Enhanced support for MyChem.info client (using `get_client("chem")` or `get_client("drug")`)
Added initial support for MyDisease.info client.
Dropped the support for Python 2.6 (might still work but we don't explicitly test it under
Python 2.6 any more)
Support for Python 3.7
When returned objects as a Pandas Dataframe (i.e. using "as_dataframe=True"),
duplicated and missing hits are now returned as dataframes as well (Thanks sdhutchins!)
Many code optimizations.

Optimized the code folder structure