Validate Glean TOML Dataset

Fixes #11

- Add glean validate command
  - `--validator-path` flag customizes validator program path
  - The first option sets dataset path and defaults to '.'
- Add `Glean::Validator` module to encapsulate validator logic
- Validate TOML dataset with tomlv program
  - If tomlv program isn't in the user's `$PATH`, attempt to install it
- Download platform specific tomlv program from [glean/glean-validator](https://github.com/glean/glean-validator) repo
  - `Glean::Installer` asks user where to install tomlv program only
    when `--validator-path` flag isn't set
  - `Glean::Installer` gracefully fails in a few cases:
    - User decides not to install tomlv program
    - Platform can't be determined or matched to tomlv program
    - Network error while downloading
- Generate new cassettes for failing specs

**OS X Demo**
![glean-osx](https://cloud.githubusercontent.com/assets/164506/2686150/c7868ee2-c1e9-11e3-88d4-0cd396d67839.gif)

**Ubuntu Demo**
![glean-ubuntu](https://cloud.githubusercontent.com/assets/164506/2686151/db80d1fa-c1e9-11e3-996b-a25dbecf12cd.gif)

cc @johndbritton @jonmagic