Not the focus
Small fixes
Fix failed unit test on Travis/gcc
Check for zero-weight constraints

Library used to crash when a constraint was added with a weight of
zero, now throws an exception. Related to #18
Nudge Launchpad
Fixed bug in required edits and stays

Fixes #18
Added linked and action variables

Fixes #15

Linked variables use an existing plain old variable as their
storage. Action variables invoke a callback function whenever they
get updated.
Fixed autosolve=false

Fixes #16
Remove asserts
std::map is slow, switch to flat_map
Convenience function for suggestion one variable
Nicer example