Final steps for MySQL support

[sgrif]

We're close enough to being done and the remaining items are small/scattered enough I'd like to document them here in case anyone wants to take a swing at them.

• Separate the PG/MySQL implementations of determine_column_type in codegen, fix the MySQL implementation to convert int -> integer, tinyint(1) -> bool, strip parens otherwise
• Support MySQL in Diesel CLI
• Enable MySQL on Travis
• Remove the cfg flag from as many integration tests as possible that are currently not running for MySQL because they modify schema inside of a transaction
• Implement timestamp helpers
• Implement prepared statement caching
• Support SystemTime and chrono

[sgrif]

#657 fixes item 1

[sgrif]

Timestamp helpers can wait for now.

[diggyk]

I know the helpers are not ready, but can timestamp or i64 be used at all at the moment? I'm having trouble with a struct that I want to use as Insertable:

#[derive(Insertable)]
| ^^^^^^^^^^ the trait diesel::Expression is not implemented for i64