New functions to create/read/write tables

A set of new functions to deal with ASCII and binary tables. Most of them are thin bindings to CFITSIO, but in a few cases I modified a bit the API:

1. Instead of one function which creates a table HDU, I implemented two of them: `fits_create_ascii_table` and `fits_create_binary_table`. I feel this is much clearer to read. Moreover, using the Tab key from the Julia REPL allows one to easily pick the right function name. This would not have been true if the binding had required to specify some string (like `ASCII` or `BINARY`) as its second parameter.

2. The way columns are specified in a call to `fits_create_ascii_table`/`fits_create_binary_table` is through the type `ColumnDef`. I believe this is significantly easier and less error-prone than CFITSIO's approach of asking *three* string arrays. See an example here: https://github.com/ziotom78/adafits#differences-from-cfitsio

The Sphinx documentation has been updated accordingly with a section named "Table routines".