Add Guard for specs
Updated README with a migration example for allowing nills

If you use the migration helper for an allow_nil field, it takes a bit of digging to work out how to do it. The normal approach of `null: false` doesn't work.

I've added a working example to the section on allowing nils to make it a bit clearer:

```ruby
add_money :products, :optional_price, amount: { null: true, default: nil }
```
Add project-specific Code Climate badge
Merge pull request #124 from remiprev/patch-1

Add project-specific Code Climate badge
Fix mongoization of Money when using infinite_precision.
Merge pull request #129 from fklingler/patch-1

Fix mongoization of Money when using infinite_precision.
.gitignore: added .ruby-version (rbenv)
README: added section with setup instructions for gem developers
Humanized_money_with_symbol: now accepts options that'll be passed on to .humanized_money
Merge pull request #131 from paulwittmann/added_options_to_humanized_money_with_symbol

Humanized_money_with_symbol: now accepts options that'll be passed on to...
Merge pull request #130 from paulwittmann/added_gemfiles_docu

Added gemfiles docu & still have trouble setting up specs =(
Add getter and spec for default bank configuration param
update broken link to active record guide
Merge pull request #137 from djbender/patch-1

update broken link to active record guide
Remove implicit usage of 'currency' as default instance currency name

This commit breaks backwards compatibility in the case of instance
currency usage. It is an aggressive decision to avoid possible clash
of names when e.g. a custom attribute 'currency' is used in a model.

It addresses issue #132
Replace instance_variables with let statemnts in monetizable spec
bump money.gem requirement to 6.0
update contributors
version bump to 0.9.0
Merge pull request #122 from mcoms/patch-1

Updated README with a migration example for allowing nills