Added support for enum values.

Added the following validations for enum values.
- is-valid-value
- has-flag
Added enum-guards, and a string guard

Added files for enum-guards, and added a string guard
Added is-one-of, and is-positive and is-negative

Added the following guards:
- is-one-of (T)
- is-positive (int/long)
- is-negative (int/long)
Merge pull request #14 from maartenoosterhoff/master

Added support for enums, and added various new guards