Delay defining SassFilesystemImporter

Since Sass is required lazily, SassFilesystemImporter should be defined
after it's loaded.
Exclude more Array instance methods from forwarding.

Fixes #244
Encoding & better place for the extracted file
Merge pull request #241 from cdlm/fix-sass-requires

Lazily define SassFilesystemImporter
Update version to 3.6.1
Add 3.6.1 release notes
Merge pull request #250 from bobthecow/unforwardable

Exclude more Array instance methods from forwarding.
Fix warnings regarding {s and }s in regexes
Merge branch 'release-3.6.x' of github.com:nanoc/nanoc into release-3.6.x
Update release notes for 3.6.1