NbConvert: Writers, No YAML, and stuff...

# NbConvert: Writers, No YAML, and stuff...

Running `ipython nbconvert` will read the default config and then apply it to one or more notebooks.
Multiple notebooks can be specified at the command line in a few different ways:
  
    ipython nbconvert notebook*.ipynb
    ipython nbconvert notebook1.ipynb notebook2.ipynb
    ipython nbconvert # this will use the config file to fill in the notebooks

## Short summary of changes

- Transformer / Filter parameters removed from constructor of Exporter and Export_...() methods 
  in favor of traitlets added to the Exporter
- Writers added
	- *FileWriter* : writes to the user's drive, either in the cwd or a subdirectory (*/nbconvert_build* by default)
	- *StdoutWriter* : writes the Jinja output directly to the stdout with no additional headers (may be useful for piping data to other applications)
	- *DebugWriter* : pretty prints the resources dict to stdout
  A custom writer can be used by setting the *writer_class* traitlet (DottedObjectName) to the namespace&name of the writer
- Ability to specifiy Transformers / Filters via namespace&name (DottedObjectName)
- Notebook names are treated as patterns and are evaluated using glob.glob
- Notebook App now inherits from *BaseIPythonApplication* which allows it to specify a default config file, 'ipython_nbconvert_config.py'
- The coupling of file-related data throughout the api was reduced by taking advantage of the resources dict.
- A ConvertFiguresTransformer and Svg2PdfTransformer were added.  ExtractFigureTransformer now extracts figures only (without the ability to renamed the figures).
- nbconvert is more verbose, including usage examples at the commandline and additional error handling.
- Resources dict was changed to a DefaultDict that returns an empty string if a key doesn't exist.
- Small things, like renaming cell_transform to transform_cell, etc...

## Resources dict

Keys
- 'unique_key' : (optional, provided by api user) key unique to the single notebook being extracted that allows extract figure transformer to give the figures unique names. 
- 'figures': contains a dictionary of figure filenames & binary data
- 'output_extension' : used by the FilesWriter to write the output with the correct extension
- 'metadata' : dict of information from the origin of the notebook (if available)
     - 'name' : name of the notebook
     - 'modified_date' : date the notebook was last modified

## Footnote

There may be some things missing in this description.
This PR needs a good reviewing.
I'll probably update this description later.

**Diffs** : Probably useless...  You may just want to checkout the branch.
**TODO** : 
- [x] Update readme.rst if necessary...
- [x] Install inkscape and test SVG conversion