Variable server urls
variable domain in docstring
load_json_dict checks if file exists and returns {} if errors.

also, save_json_dict checks if what it's saving is a dictionary.
Added CONFIG_FILE, test for file-read/write permissions.

Credentials are separated from config items.
updated plotly.py to use new syntax and tools.get_config_file().

(also deleted some things that were commented out)
raise plotly error if users try to write to files without permission.
Fixed old reference to _get_plotly_urls
Added circle test for write-permissions in .plotly directory.
Edited circle.yml to lock .plotly folder and test import.
Made warning message nicer about file permissions, added dir check.
Merge pull request #24 from plotly/variable_server_endpoint

Variable server endpoint