should refactor the validators hack for table schema url format fields

[OriHoch]

there is a problem with the json schema validator and the handling of file paths in the resources data property.

• the data field is defined as array of strings with url format
• however, datapackages allow relative file names
• the json schema validator identifies it as a validation error

currently it was fixed with a simple hack that prepends file:// to the data field

need to review and look for possible problems with this hack, then fix it properly

also, it should really be inside the datapackage class to allow extensibility

[OriHoch]

this problem was introduced in #13

[pwalsh]

@OriHoch is this still a problem with the latest spec changes (rc2)? cc @roll

[OriHoch]

this is still a problem, I opened a PR on specs: https://github.com/frictionlessdata/specs/pull/480

[OriHoch]

fixed in v0.1.4 - removed the hack as it's not neccessary for latest spes