Differentiate between ALLOCATED and FREE pages

Previously pages were returned in space-page-type-regions and
each_page_type_region purely based on the type noted in the page
stored on disk without regard to the extent descriptor's status
for the page. Now we treat pages which are marked free in the
extent descriptor as "FREE" regardless of their type in the actual
page data (since it may be stale).
Use "FREE (<type>)" instead of "FREE" in regions

In order to still provide the type info for freed pages, return the
string "FREE (<type>)" instead of just "FREE" for freed pages.
Page number in fseg entry may be undefined
Add tags file to .gitignore
Bump version to 0.7.12
Add programmatic access to page type descriptions

Move the page type descriptions that were previously in comments into
Hash members so that they are programmatically accessible. Also, clean
up the descriptions and add a usage description for each of them.
Stupidly changed the value of UNDO_LOG

Change the value of UNDO_LOG back to 2.
Only increment position by 2 in redundant clustered indexes

We only need to account for TRX_ID and ROLL_PTR in clustered indexes,
not secondary indexes.
Add methods to return page numbers of important pages

Rather than hard-coding page numbers of important space pages, add
methods for each of them, with nice names.
Finish adding data dictionary tables

Add definitions for the rest of the data dictionary tables, as well as a
convenience method for opening a data dictionary index.
Major overhaul of the RecordDescriber system

Complete rewrite of the RecordDescriber to make a much more useful
system. See one of the example record describers.
Add example RecordDescriber for employees_db sample database
Print key correctly for NODE POINTER records in index-recurse
Add a new space-page-type-summary mode

Add a new space-page-type-summary mode which prints page type and page
count (along with % and a description of the page type).
Bump to version 0.8.1
New Innodb::Record class

Records are now returned in an Innodb::Record container. This allows for
adding convenience methods for working with records.
Add "Hello World" example data file and describer
Lots of cleanup and new Innodb::System class