Make fluidity build with petsc 3.5

PETSC_DEFAULT_DOUBLE_PRECISION changed to PETSC_DEFAULT_REAL
KSP/PCSetOperators no longer have a MatStructure flag
As before, previous versions (3.1-3.4) are supported through macro hacks in include/petsc_legacy.h

Fixes #17
Merge branch 'master' into fluidity-core/lift-velocity-bcs