Created basic SDPNode class structure with validation of values.
Cleaned up 'value' variables in SDPNode to be better named to avoid variable shadowing. Cleaned up spec for SDPNode. Added to_s method for SDPNode.
Renamed SDPNode to SDNode to conform with SDP specification.
Renamed sdp_value to sd_value in SDNode.
Added YARD to gemspec. Cleaned up documentation for SDNode.
Merge branch 'feature/core' into develop