Added round option

ip_sub accepts receipts as args

Ignoring price keys when only one hash contains them

Updated ruby + gems

Reimplemented Sub#evaluate (again)

Removed possibility to pass multiple args to ip_merge

Reimplemented inverted fuction

Object#ip_sub is now deterministic

Added specs for #ip_sub and #ip_merge

Added ruby-gemset and -version files, deleted .rvmrc

Removed whitespace

Changed terminals.

No direct instance references. . Using attr_reader.

Added .rvmrc to gitignore

mit licence