Reader parses values written prior to encoding

With the addition of setting encoding on write (yay!), it is possible
that existing persisted statements were written without encoding. When
attempting to read those previously written statements, the reader
raises an Encoding::UndefinedConversionError.

By catching that error and sending it through the unescape sequence,
it is possible to apply the unescape/encoding correct behavior.

Should the RDF::NTriples::Reader.unescape method be extracted as a
service method on RDF? Given that this pull request is setting up a
case where the Ancestor class is calling a method on a Descendant
class.

This appears to be an issue when moving from 1.0.9 to 1.0.10

https://github.com/ruby-rdf/rdf/compare/1.0.9...1.0.10, in particular
https://github.com/ruby-rdf/rdf/compare/1.0.9...1.0.10#diff-94c44c6c4ab4c6e92f428841d0e827c3L60
Merge branch 'read-previously-written-values' of github.com:jeremyf/rdf into jeremyf-read-previously-written-values
Solutions Enumerator improvements for Ruby 2.0
Fix issue with Solutions#offset where starting offset as greater than the size of the enumerator.
Update N-Triples/N-Quads to support end-of-line comment.
Add some NQuads test cases.
Fis Solutions#limit for JRuby.
Fix frozen string issues with encode!
This resolves issue #138
Fix RDF::Statement to either return <(http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement> if no arguments are given, or an insteance of RDF::Statement.
Merged 'develop'.
Finish NTriples
When sampling, force encoding to ASCII_8BIT instead of US_ASCII.
Use regular array when iterrating in Query#execute, rather than trying to use a Solutions::Array.
Optimize Solutions#empty? Solutions#project takes a block or returns an Enumerator.
Minor fix in Queryable#query_execute to call query.execute with a block, rather than running each on the result.
Update README with GitFlow information for contributors.
Merged 'develop'.
Refactor strict vocabularies to generate vocabulary files directly, including property taking name, label and comment.
Update all vocabularies as generated static vocabularies, except for xsd.
This completes the refactoring of issue #132