Add specs for ENV#set including failing specifications for Ruby "basic types" implicitly converted by YAML

This commit addresses issue #13.

Since Ruby's implementation of YAML has a set of ["basic types"](http://www.yaml.org/YAML_for_ruby.html#basic_types) that are implicitly converted from String to the appropriate type, Figs should convert the "basic types" to strings and store them in ENV, so that a calling application can just call ENV['VAR_NAME'] for "basic types", which is more intuitive.

Basic types handled by YAML and included in the specifications for ENV#set:
- Integer
- Float
- Null
- Boolean
- Time (in the ISO8601 format)
- Date (in the ISO8601 format)