Improve validation messages

[fredbi]

We should improve the quality of messages reported by the validation.
In particular, we should be able in one single validation pass, to report about most known errors.
Also, validate should more resilient to spec errors and avoid panicking as much as possible.

Fix the following issues

A number of issues have already been posted about this.
This issue recaps the ones that might be short term improvements.

• Issue Pattern function could probably store compiled patterns. #16 - Cached compiled regexp
• Issue validation message list contains duplicates #12 - Multiple instances of messages
• Issue Enhancement: multiple instances of error messages for invalid spec go-swagger/go-swagger#1307 - Multiple instances of messages (duplicate of validation message list contains duplicates #12)
• Issue dot symbol in the path cause problems go-swagger/go-swagger#1050 - Variant in path params (e.g. dot separated)
• Issue $ref in headers causes "forbidden property" error on validation go-swagger/go-swagger#1243 - Explicit error for $ref within headers
• Issue Validation issue: There is one validation error for in the parameters item. go-swagger/go-swagger#161 - parameter items validation (a default value check)
• Issue swagger validate: the need for better error messages go-swagger/go-swagger#859 - Richer message for unresolvable $ref
  (better than "object has no key").

Add the following validation features

(not in other existing issues)

• Improve checks on garbled path params ({x/y} , ..)
• Check for path param uniqueness
• Check path params are declared as required
• Optional "continue on error" mode, to allow for checks to continue and sweep through all
  detected errors.
• Replace regexp.MustCompile based on external output by simple compile and error management
• More unit testing, with stricter assertions on errors, warnings and their numbers
• Add check on multipleOf factor which must be positive

[fredbi]

This list should be solved with PR #42