Add Columns endpoint

[mathemancer]

Fixes #199

This adds a column API to perform CRUD operations on table columns. These can be accessed via:

http://server.address/api/v0/tables/<table_id>/columns/
http://server.address/api/v0/tables/<table_id>/columns/<column_idx>

Here, <column_idx> is the index of the column in the column list associated with the table in question (indexed from zero).

Technical details

• POST requests to add columns need at least name and type keys. Optionally, one can also use nullable (this takes a boolean).

Note that a DELETE which removes a column from any place except the last will result in indices of columns to the right of that which was removed decrementing by one.

Screenshots

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[eito-fis]

Some small questions about handling 404s, but overall looks good to me!

[eito-fis]

Should we also catch an IndexError here, like in retrieve()?

[mathemancer]

Good idea, will do.

[eito-fis]

Same as above, should we catch IndexError?

[kgodey]

@mathemancer Looks great! I added a comment. I'll leave it up to you to fix and merge.

[kgodey]

Can't you rename the records_router to table_router or something like that and register both the records and columns to the same router rather than making a new one?