Add unary - and unary + to cypher parser

Resolves #1213
Moved constraint index validation into prepare phase of constraint creation.

This removes a race condition where changes could occur between validation and
commit. Now we are protected by the synchronization on prepare/commit.
Merge pull request #1214 from cleishm/unary_plus_minus

Add unary - and unary + to cypher parser
Allow arguments starting "--" for shell commands that don't take options.