Support entering datapoints as fractions

[theospears]

Many non-integer data points are more naturally entered as a fraction than a decimal. For example reading 1/3 of a paper, or eating 1/2 of an Apple. The beeminder website facilitates this by allowing entering fractions in the data entry fields, which are then converted to decimal.

Request:

• Support entering data in the form x/y in the beemios value inputs
• Add / as a button to the keyboard on iPhone
• Convert this to a decimal value before submitting

Notes:

• Values will be converted and then displayed as decimals after being entered - this is purely an aid for recording values.
• We shouldn't support combining this with : for time inputs. One or the other!