Add labels to the metadata in user files

All lines in a user file should be prefixed with the type of information on that line. The goal is to make reading and updating user files a little easier for the humans involved, in addition to making them a little more robust and powerful for the parser.

Example:

```
email jweacho1@binghamton.edu
section a0
grade assignment01 10 10
grade lab01 10 10
```
*Note: there should be a __space__ between the label and the data, but there should be __tabs__ in the `grade` values.*

Also fix a bug where editing your email would erase all of your grades, because `user.write()` never wrote grades back into the file.

#### Migration

Hopefully this should update all existing user files: [`migrate.py`](https://gist.github.com/6545242)