Parsley 3.1.0

This release incorporates the changes in #92, #93, and #94. In summary, added the <+> combinator, and amend and entrench in parsley.errors.combinator. The calling mechanism for recursive parsers has been reworked and should be better for performance and memory consumption for larger parsers. The error builder now supports multiple lines of error input, with the default set at 1 line before and 1 after. This can be configured by extending the error builder class. This change has shifted the revision number of the builder to Revision1 and Revision0 retains the original behaviour of a single line.