Defer loading element data until attribute access

[paulromano]

Here's my quick attempt to reduce the overall import time by deferring the autoload of element data until one actually accesses the corresponding attribute from the mendeleev module, e.g. mendeleev.Li. This seems to work pretty well in my limited testing. The only downside I can see is that the attribute names (H, He, etc.) won't show up in an autocomplete from an IDE. @lmmentel Let me know what you think of this approach.

Fixes #118