Code cleanup: switch to more LUT's
Code cleanup and add some inline documentation
Move trace_level helper functions out of loop
Move constants out of trace_contour
Change local declaration to explicit assignment

Using the local keyword to declare a variable did
not seem to work on Travis CI. The variable is
now explicitly assigned a 0 value
Merge branch 'master' into dev
Split add_vertex into interpolate and add_vertex!
Move `interpolate` out of `trace_contour
Merge tests from master to dev
Fix infinite loop from data with only 4 vertices
Fix tests and exit_face for cell case 18
Fix input convention (r,c)->(x,y) to close #2
Change output format as discussed in #1
Fix compatibility issue

In Julia 0.2, `ContourLevel(h, [])` does not seem to
automatically infer the array type.
Add restriction that z <: Matrix{FloatingPoint}
Update tests to follow z::Matrix{T<:FloatingPoint}
Make return type of `interpolate` more generic

The conversion to a `Vector2` type is moved to `add_vertex!`
Moved chase out of trace_contours
Removed unused and unexported methods
Add some default constructors to enable looser type constraints