Improved MySQL support

[smaccona]

Adds support for utf8 and utf8mb4 character sets in MySQL's default
configuration.

Closes #38.

[smaccona]

@avelis if you can review when you get a chance. Here are the 3 scenarios I tested for locally:

• Fresh install: works
• Upgrade over existing install on proper MySQL configuration with long index support: works, does not change underlying fields (they stay at width 300)
• Upgrade over modified install on MySQL with no long index support (user manually changed the field width or created a truncated index and faked the initial migration): works, does not change underlying field (they stay at whatever width the user had them)

[avelis]

@smaccona Thanks again for working on this PR and long standing issue. Much appreciation from me.

I am planning a release with a major version bump. I will make sure to set the expectation this will break current installations. However, the upgrade scenario's you noted might work for others and I will highlight them.