Support foreign key checks

[roll]

• improved validate
• connects Support PK/FK for integrity checks datapackage-js#82
• added additional describe level to Table tests (diff just because of indentations only FK tests are new)

---

On tableschema level we just accepts references argument for Table constructor. It's because tableschema layer is not responsible for integrity checks by itself but provides tools for it like schema.foreignKeys and table FK checks if references is passed.

So on datapackage level those references should be composed. See this PR tests and readme diff for more references argument information. See datapackage implementation - frictionlessdata/datapackage-js#89

[roll]

@amercader
Could you please review?

[amercader]

+1

[amercader]

Maybe add a test for self reference FK, as in the Python implementation? Can be done on a separate PR though.

[roll]

We have for self-referencing in datapackage tests.

[amercader]

True, sorry I got the two mixed up!

[Stephen-Gates]

@roll thanks so much for this change. It really helps my project and your timing is perfect.

I'll think about the wording to use in spec.json for the new error message you added to support #86

[roll]

@Stephen-Gates
I'll suggest fixing #86 on Aug 4 sprint planning.

[Stephen-Gates]

@roll here's my attempt at an error message.

message: The value {value} in row {row_number} and column {column_name_A} does not have a matching value in column {column_name_B} in table {table_name_B}

description: A matching value should exist for a valid foreign key relationship. How it could be resolved:

• If this value is not correct, update the value.
• If value is correct, then add it to column {column_name_B} in table {table_name_B}
• If this error should be ignored, then disable the foreign key check for column {column_name_A}

Not sure if the last dot point is needed - just conforming with format of other messages.

[roll]

@Stephen-Gates
Thanks! I've add it to #86