Added LinearRing import statements

Added LinearRing and asLinearRing to import statements in order to make
loads(wkt) work for LinearRings.
Note that GEOS development headers are also required for speedups.
Merge pull request #58 from maarten-vermeyen/loads_linear_ring

Added LinearRing import statements
Merge pull request #61 from JasonSanford/speedups-docs

Note that GEOS development headers are also required for speedups.
Implement set operators for base geometry
Tests for operators
Fix operator test
Merge pull request #62 from kjordahl/master

Implement operators analagous to uses by builtin set type
Merge branch 'master' of github.com:Toblerity/Shapely
Updates to manual and change log for 1.2.18.