Remove useReferences from SchemaOptions

[fizruk]

Closes #21.

[fizruk]

@dmjio can you review this, please?

[fizruk]

I will try to outline the changes in this PR, since the code is not very self-explanatory.

The goal of this PR is to

• remove useReferences option from SchemaOptions and
• always use references for generic schemas;
• provide means to inline any schema references as an after effect.

Since schema references appear in different data structures I am using Data.Data.Lens.template to traverse any Data and inline references. This is particularly useful for inlining schemas globally for Swagger spec (e.g. it will inline schemas used in definitions, parameters and responses too).

I have added some documentation here to make it clear what different helpers are for.

In order for generic inlining to work, we need Data instances for all the types contained in Swagger. Although it can be derived for most of the types, some I had to write manually:

• Host, MimeList, Example — all use types with hidden constructors;
• SwaggerType, CollectionFormat — Data can't be derived for GADTs.

@dmjio these Data instances are what I'd like to be reviewed (since it's the first time I deal with Data).

There are also two minor changes in this PR:

• time schemas don't have minLength property anymore since I think it's actually useless;
• Travis now builds swagger2 with -O0 for a considerably faster builds; I suspect the slowdown was due to optimizing Generic and Data code (and probably TH code); if you happen to know a better way of doing things, I am open to suggestions!

[fizruk]

OK, I have played with Data instances, made some improvements and tiny fixes.

I am going to merge this PR now.