add checkDependency for student snopt
fix a bug in PolynomialProgram, that it cannot actually set solver
1. zap the separate check for snopt and student snopt in checkDependency
2. add lsqlin in checkDependency
Merge branch 'master' into add-snopt-student

Conflicts:
	systems/plants/constraint/QuasiStaticConstraint.m
Merge pull request #426 from hongkai-dai/add-snopt-student

add checkDependency for student snopt