Add support for `private_constant` class method calls to recognize private
class, module and constant definitions (proposed for Ruby 1.9.3).

Closes gh-219
Add support for `private_constant` class method calls to recognize private
class, module and constant definitions (proposed for Ruby 1.9.3).

Closes gh-219
Make sure the non-tag form of (see ...) only works at the start of a docstring
Add specs to thread local storage code, improve initializations
Adds threadlocal behaviour to the Registry class
so that it can be used across multiple threads in
a threadsafe fashion.

Closes gh-221
Fix formatting in spec
Initial single object db implementation
Allow disabling of single_object_db
Add check for empty object db
Add spec for single_object_db attribute default value
Add specs to single object db functionality
Add support for storing objects to a single db
instead of multiple files on the filesystem, for more efficient filesystem
usage.

Adds switches --single-db and --no-single-db to the `yardoc` command, and
YARD::Registry.single_object_db for programmatic control of whether a single
object db should be used.

Closes gh-222
Fix documentation on tag library
Make SimpleMarkupHtml thread local, remove constant
Add documentation to simple_markup_html
Split index/listing templates into listing/files/objects
Fix broken specs
make sure that alternatives specified in @deprecated don't get struck through and that they're set using a monospace font
Update default_factory_spec.rb formatting
Remove trailing whitespace from freeform tags

Closes gh-229