Add :tolerance opt for "close" numeric comparisons

There is often a requirement to compare numeric values within a given
epsilon - for example, 45.0132 should be "equal" to 45.013, given a
tolerance of 0.001.  This is especially useful when comparing the
results of financial calculations.
Add :tolerance documentation to README

Also cleaned up some existing documentation.
Allow custom compare method (or built-in filters)

Now you can specify a custom comparison method to use when comparing
values within the Hashes or Arrays being diff'd.  By default, #== is
still used.  You can also specify a hash of built-in filters, in which
:numeric_tolerance and :strip are currently recognized.

Also, more documentation cleanup, and make comparison less restrictive
when comparing numeric types.
Default strict numeric compare, option to turn off

Revert to comparing numeric types as well as values (strict comparison),
to maintain expected behavior and not break API.  Add :strict option
which defaults to true but can be set to false to allow for looser
comparison of numeric types (e.g. Fixnums can compare to Floats).
Merge pull request #4 from ravigadad/custom-comparisons

Allow custom comparison methods, with a few optional built-in filters