Created documentation index page for new API reference manual.

The API Reference will be moved into its own document (and out of the
Developer's Guide).
Always show members and inherited members for autodoc entries.

These are almost always included, so we turn them on by default. You can
turn them off for individual entries using :no-members: and :no-
inherited-members:, respectively.
Added rmgpy.thermo section to API Reference.

The organization is based somewhat on that used by scipy. The index page
contains an overview of the classes and functions defined in the subpackage,
with links to separate standalone pages for each class and function. The
toctree is hidden in favor of an alternative (table-based) means of
specifying the classes and functions.

You can still supplement the in-code docstrings with extra content, but note
that this will appear after the in-code docstring in the generated HTML.
Added rmgpy.kinetics section to API Reference.

Again following the template of the section for the rmgpy.thermo module.
Added rmgpy.statmech section to API Reference.
Added rmgpy.constants and rmgpy.quantity sections to new API Reference.

Doing these together since they were grouped in the Developer's Guide.
However, in the API Reference I want each module and subpackage of rmgpy to
have its own section, so they are split up here.
Added rmgpy.molecule section to API Reference.

One of the more complex subpackages; hopefully this organization is useful!
Added rmgpy.reaction and rmgpy.species sections to API Reference.
Added rmgpy.pdep section to API Reference.
Added rmgpy.data section to API Reference.

By far the largest and most complicated subpackage.
Added rmgpy.solver section to API Reference.
Added rmgpy.chemkin section to API Reference.

The functions in this module are divided into those that are used for
reading Chemkin files and those used for writing Chemkin files.
Added rmgpy.cantherm section to API Reference.
Added rmgpy.rmg section to API Reference.

The last subpackage!
Removed all references to API from Developer's Guide.

They have all been moved to the API Reference.
Small tweak to the documentation CSS.

To ensure there is a bit of padding at the bottom of the main content area,
particularly when the last item in that area is a table.
Assorted updates to the code to fill in the API documentation.

A few missing docstrings were added. A few imports were also added to the
__init__.py of a few subpackages to provide more convenient access to those
classes.
Another tweak to documentation CSS for wrapping long words.

Some of the titles in the API Reference are long words with no breaks.
Without this change, those words would bleed out of the sidebar and into the
main text area.
Added table with module overview to API Reference main page.

The table shows a brief description of the main purpose of each component
(module or subpackage). You can click on the component name to view more
information about that component.

With this commit, we have completely hidden all toctrees in the API
Reference in favor of the manual, table-based navigation. This perhaps
requires a bit more effort to maintain, but I think it's a cleaner
presentation of the API overall.
Merge branch 'documentation'