Remove jsontableschema dependency

[roll]

It fixes #51

Instead of having processors.Schema(schema) now we have processors.Convert(converter) to convert string to python objects. It could be used also with JTS:

schema = jsontableschema.Schema(descriptor)
topen('path.csv', processors=[processors.Convert(schema.convert)])

[roll]

@georgiana-b
could you please review

[coveralls]

Coverage increased (+2.2%) to 91.215% when pulling af5d1fe on remove-jts-dep into 65746c1 on master.

[coveralls]

Coverage increased (+2.2%) to 91.215% when pulling af5d1fe on remove-jts-dep into 65746c1 on master.

[coveralls]

Coverage increased (+2.2%) to 91.215% when pulling e596983 on remove-jts-dep into 65746c1 on master.

[coveralls]

Coverage increased (+2.2%) to 91.215% when pulling 2278647 on remove-jts-dep into 65746c1 on master.

[coveralls]

Coverage increased (+2.2%) to 91.152% when pulling 1470a1d on remove-jts-dep into e15c331 on master.

[coveralls]

Coverage increased (+2.2%) to 91.152% when pulling b475280 on remove-jts-dep into e15c331 on master.

[coveralls]

Coverage increased (+2.2%) to 91.152% when pulling b475280 on remove-jts-dep into e15c331 on master.

[coveralls]

Coverage increased (+2.2%) to 91.152% when pulling b475280 on remove-jts-dep into e15c331 on master.