pyproj.crs updates/fixes (is_proj/to_proj4/to_dict/from_string)

[snowman2]

• Tests added
• Fully documented, including history.rst for all changes and api/*.rst for new API

Fixes summary:

• added 'is_proj';
• fixed case in 'to_dict' with 'to_proj4' returning None
• keep no_defs as it does not hurt anything in current code and it preserves backwards compatibility with older versions
• made public properties on C classes readonly

ping @djhoese