Add sampling frequency options

[spestana]

Addressing #31

I've added a sampling frequency options of 1 minute (instantaneous samples), 5, 15, 30 minutes, and 1 hour (averaged over that time period). I did this by adding an optional resampling step within _db.get_sensordata(), and a second dropdown menu on the dashboard. The sampling frequency option selected is also reflected in the data.csv download. I also changed the default duration and sampling to 7 days and 1 hour respectively, just because I thought it looked more interesting than the "noisy" instantaneous data.

Other minor changes in this pull request:

• cosmetic changes to map dashboard (map width, dropdown menu labels, map marker size)
• dataframe key cleanup (removed subscript and degree symbol from dataframe column names)

[spestana]

[keenanjohnson]

Love it!