Lambda-based find()

API changes:
- defined new find(text, pattern, \[\](){}, Algorithm()) signatures - requires C++11 to accept non-const rvalues
- renamed delegate-based Finder2 classes as Finder_ and all their related functions as _...()

Algorithmic changes:
- added Backtracking\<Exact\> algorithm for exact search
- added Backtracking\<EditDistance\> algorithm, based on simple neighborhood generation

This PR is required to fix a MSVC bug in Yara.