Rename default quality class methods

- New methods: Epeg::Image.default_quality and
  Epeg::Image.default_quality=
- Remove methods: Epeg::Image.quality and
  Epeg::Image.quality=
Fix issues when resizing beyond image bounds

Modifies Epeg::Image#crop(w, h, [x, y]) s. t.:
w, h, x, or y exceeding the image dimensions are truncated accordingly
Clean up

Epeg::Image.initialize
- Remove unused variables
- Correct signedness for width and height

Epeg::Image#to_blob
- Correct signedness for data pointer
Add valgrind support

- Spec runner: spec/runner.rb
- New rake valgrind task to actually run valgrind and check for
  memory leaks
Fix memory leak

EpegImage closes correctly now:
- Epeg::Image#write
- Epeg::Image#to_blob
version bump
Merge branch 'v0.1.2'