WIP documentation for 2.2 release

[alimanfoo]

Main TODOs...

• rework docstrings for storage classes to show how to use with array or group
• add tutorial section on using DBMStore (resolves Add persistence examples using shelve/dbm, lmdb #131)
• add tutorial section on options for string (object) arrays (resolves Support string/byte arrays #135, resolves DOC: string (object) arrays #175)
• add tips subsection on support for pickle (resolves DOC: Pickle support #91)
• review docs on saving to disk (resolves Docs: Exporting existing NumPy array to disk #101, resolves DOC: Integrate docs on saving existing array to disk #103)
• document chunks=True (autochunking) in tutorial (resolves DOC: chunks=True #107)
• Fix specification of datetime (resolves Specification for datetime dtypes is ambiguous #85)

Also need to add release notes (resolves #159) for enhancements:

• advanced indexing
• tree viewer
• NestedDirectoryStore
• DBMStore
• numcodecs
• ...

...and other work...

• changes to storage spec (fill value encoding, datetime spec, ...)
• ...