Added a new fast reading/writing engine to io.ascii

This PR adds a Cython/C parsing engine to the infrastructure of `io.ascii` so that new reading classes for simple formats (currently `FastBasic`, `FastTab`, `FastCsv`, `FastNoHeader`, `FastCommentedHeader`, and `FastRdb`) can opt to use this engine instead of the ordinary `BaseReader` inheritance model. The ordinary readers for these formats still remain to maintain flexibility and compatibility. The PR introduces a new parameter ``use_fast_reader`` in `ascii.read()` which can be used to enable or disable the fast engine (by default, `read()` will attempt to use the fast reader and fall back on the slow reader if parsing fails). The fast readers are also on the guess list directly in front of their slower counterparts.

There is also a Cython engine for writing with these formats; a similar parameter ``use_fast_writer`` can enable or disable this engine, and `ascii.write()` uses the engine by default when writing output in a compatible format.

Using randomly generated text files, it seems that the fast reading engine is anywhere from 3 to 7 times faster than the ordinary readers depending on the data type of table columns, while the fast writing engine is anywhere from ~2.5 to 12 times faster than the ordinary writers; both engines have less of a speed gain for floating-point data, which might be worth looking into at some point. [Here](http://amras1.github.io/astropy-benchmarks/#) are some before and after benchmarks, which cover a range of ASCII formats and compare Astropy's performance with that of numpy and Pandas. There is also a short design document [here](https://github.com/amras1/astropy-benchmarks/blob/master/design.md), which gives a quick overview of the parsing implementation in this PR.

Please feel free to make any suggestions or ask me anything.
Hmm, good question.  The easiest thing is just leave it on @amras1's github repo (i.e. right where it is), and just leave this reference.  

A slightly more complex option would be to make a new astropy repository for notebooks like this.   To go further with that, if we want auto-build behavior, we can make it a project for someone to convince travis to work as a build-bot, or convince someone to set up a machine that automatically re-builds them and sends them up to the repo when a web hook is triggered from astropy master.

But if you think it makes sense for this to be in the source code distribution, it could also go in the docs directory - that doesn't all *have* to be rst.  But then there's no way I know of to auto-build it, though.