Add support for a default country code

[jerryclinesmith]

Instead of always passing country code on each call, add support for it to be set once (probably in an Initializer):

PhonyRails.default_country_code = "US"

[coveralls]

Coverage decreased (-0.2%) to 99.758% when pulling 6465ad6 on jerryclinesmith:default-country-code into cfb90f5 on joost:master.

[coveralls]

Coverage remained the same at 100.0% when pulling ce7576c on jerryclinesmith:default-country-code into cfb90f5 on joost:master.

[joost]

@jerryclinesmith looks good! Can you add some info in the README?

[jerryclinesmith]

Comments added.

[coveralls]

Coverage remained the same at 100.0% when pulling 384bab0 on jerryclinesmith:default-country-code into cfb90f5 on joost:master.