Refactor IO API for consistency and symmetry

[glopesdev]

Similar to other numeric and data frame processing libraries we would like to minimize the redundancy of functions exposed by the io module to avoid confusion.

Here we merge read and parse into a single read function accepting either a file-like or a buffer-like object. For consistency and symmetry, we also rename write and format as to_file and to_buffer functions, respectively.

Finally, the order of the first parameters into the to_file function has been swapped to match what would be more closely expected from an extension method.