Move `convertText` to `typeConverter`

Create a new module `lib/type-converter.js`.

Move `routeHelper.convertText` to this new module.
Refactor conversion of data types

Refactor the way how loopback types are converted to swagger data types.

 - `modelHelper.LDLPropToSwaggerDataType` is responsible for
   producing a valid Swagger Data Type object from LDL object
   (be it a property, accepts item or returns item).

 - LDLPropToSwaggerDataType picks only fields that are part
   of the swagger spec, everything else is excluded from the result.
   It's up to the caller to add extra fields like `description`.

 - refactor `routeHelper.extendWithType` to accept an additional arg:
   the original LDL object. This way it's possible to copy all
   type-specific fields to the output object and don't add anything
   else.
models: include model's `description`
Merge pull request #64 from strongloop/feature/property-constraints

Preserve property type constraints