adds more flexibility to defining endpoint urls

- symbols with underscores will be converted into paths
  `Hyperdrive::Resource.new(:goof_ball)` => `/goof/balls`
- Hypedrive::Resource now takes an options hash. Currently only supports
  `endpoint` which will allows you to set any url you want.

refs #24
Merge pull request #25 from styleseek/24-endpoint-urls

adds more flexibility to defining endpoint urls

fixes #24