Fix README, add .gitignore
Implementing timeout for requesting Graphite URL
Basic exception handling
Check data freshness
Fix logic error when getting graphite data -
`bool(eval("0.0"))` returns `False`, so the conditionals here failed if
`all_data_points` was like `["0.0", "0.0", ...]`.
 Replace call to `eval` with explicit check for `"None"`, because eval is dangerous.
Clarify the -U and -L apply only with -W.