Change StatusCode to handle unknown status codes

[jbearer]

Closes #229

This PR:

Tide's design of their status code type (which we copied) as an enum, is not resilient against new status codes or status codes unknown to the server framework (such as ELB-specific status codes). This caused a panic in a Cappuccino node when it received a response with a status code it did not recognize.

This changes StatusCode from an enum to a wrapper around reqwest::StatusCode, which is itself a wrapper around a u16. As such, it can represent any status code in the allowed range without necessarily knowing what a certain status represents.

We keep most of the same interface by reimplementing all the appropriate and conversion traits. However, there are a couple of breaking changes:

• We no longer implement From<StatusCode> for tide::StatusCode; instead we implement TryFrom, so we can return an error instead of panicking if it is a status code that tide cannot handle
• Since our status code is no longer an enum, we replace enum variant constructors with associated constants, which is the same as what reqwest does