Metadata cleanup

This commit:

* Adds a Gemfile so that Bundler can be used to install dependencies.
* Puts Gemfile.lock in .gitignore, since that file shouldn't be in source
  control.
* Specifies Shoulda as a development dependency in the gemspec file.
* Converts tabs to spaces in the gemspec file.
Merge pull request #15 from reinteractive-open/metadata_cleanup

Metadata cleanup
[Fixes #9]: Explicitly initializes the places where Flt::DecNum is expected rather than relying on to_d
[#9] Wrap all #to_d calls with Flt::DecNum.new so we don't have type errors
[#9] Documentation updates that work within a Rails environment
Oops, README isn't markdown
minor: whitespace
avoid using octal constants

Using "01" is unwise, since using a leading zero indicates an octal
(base 8) number.
ignore vim swaps
ignore .bundle
use 2.0.0 with rbenv
converted to minitest/spec instead of test/unit with shoulda
refactored tests to use a common helper for loading
oh yeah! failing tests with active support, now to fix ;-)
tests passing now, but no more interval convenience
Tests passing with ActiveSupport, updated README
use to_s instead of to_d for rails support
Merge pull request #21 from bramswenson/rails_support

Rails support [Fixes #9] [Fixes #17] [Fixes #20]
Removed some 'assigned but unused' variables.
Merge branch 'avoid-octal' of https://github.com/xpe/finance into xpe-avoid-octal

Conflicts:
	test/test_cashflows.rb

Removed octal constants while keeping minitest structure.