add support for serde_repr

[ThouCheese]

This pull request adds preliminary support for serde_repr by adding a corresponding JsonSchema_repr macro as we discussed in #35. I say preliminary because it throws away all info encoded in the variable names. Ideally, when JsonSchema_repr is on an enum with for example:

#[derive(JsonSchema_repr)]
enum StatusCode {
    /// The process exited successfully.
    Success = 0,
    // variants ...
}

The documentation should say "* 0 - Success: The process exited successfully." I'm not too sure how to go about doing this however, so any guidance would be appreciated :).

Also, this is my first time working with the proc macro api, so I checked everything and it seemed to report okay error messages, but if there is something wrong, please let me know.

[GREsau]

This functionality has now been implemented in #76