PR: adding a core.config module to hold package-wide configurables
PR for #2081
## Summary
- core.config is a new module which serves as a general mechanism for working with configurables.
- superceds fmt.print_config, while remaining backward-compatible (set_printoptions(), reset_printoptions() are still working).
- adds the following user API's:   `get_option()`, `set_option`, `reset_option()`, `reset_options()` and `describe_option()` (all under the pandas top level module).
- adds the following for developer use: `register_option()` and `deprecate_option()`.
## TL;DR description

```
ENH: Add core.config module for managing package-wide configurables

The config module holds package-wide configurables and provides
a uniform API for working with them.

Overview
========

This module supports the following requirements:
- options are referenced using keys in dot.notation, e.g. "x.y.option - z".
- options can be registered by modules at import time.
- options can be registered at init-time (via core.config_init)
- options have a default value, and (optionally) a description and
  validation function associated with them.
- options can be deprecated, in which case referencing them
  should produce a warning.
- deprecated options can optionally be rerouted to a replacement
  so that accessing a deprecated option reroutes to a differently
  named option.
- options can be reset to their default value.
- all option can be reset to their default value at once.
- all options in a certain sub - namespace can be reset at once.
- the user can set / get / reset or ask for the description of an option.
- a developer can register and mark an option as deprecated.

Implementation
==============

- Data is stored using nested dictionaries, and should be accessed
  through the provided API.

- "Registered options" and "Deprecated options" have metadata associcated
  with them, which are stored in auxilary dictionaries keyed on the
  fully-qualified key, e.g. "x.y.z.option".

- the config_init module is imported by the package's __init__.py file.
  placing any register_option() calls there will ensure those options
  are available as soon as pandas is loaded. If you use register_option
  in a module, it will only be available after that module is imported,
  which you should be aware of.

- `config_prefix` is a context_manager (for use with the `with` keyword)
  which can save developers some typing, see the docstring.
```

should `describe_option` print the description, rather then return it as a string? 
 rebased to clear merge conflicts from PEP8 tsunami.
 The equiv. of f2ded9a should be considered for cherry-pick whether this goes in or not.
IPython finally fixed the corresponding issue on their end: ipython/ipython#2500
 This definitely will be going in in the next major release
 groovy.
 Putting a note here to remind myself to move the plot options into this module after 0.9.1
 - cleared merge conflicts against git master
- changed `describe_option` to `describe_options` : 
  - when invoked with no argument , will print help on all registered options.
  - when given an argument, print description of exact key if it exists, otherwise
    print descriptions for all option keys that match.
  - added optional argument to return description as string rather then printing it. mostly for testing.
- nicer formatting
- some cleanups
 removed 2.5 support
 Am I correct that this should be backwards-compatible change?
 I think so.
`set_printoptions` still works, even though it now relies on  core.config behind the scenes.
 Do you think this should be mentioned in the "what's new?" page (docs/source/v0.10.0.txt)? 
 Better mention the new API functions I think, that matters to users.

Will sphinx pick up the doctsrings automatically, or do I need to add docs myself? I neglected 
that part of things.
 You can add entries to the API page. But it might make sense to write some brief exposition on config options-- printoptions might be in the docs already but I don't remember adding them myself.
 updated docs/source/v0.10.0.txt
 @wesm, I think having  both `reset_option` and `reset_options` is silly in retrospect,
when is 0.10.0 coming out? I'd like to mend this before the API becomes part of a stable release.
