Added support for custom _id-s

As per issue #154, if the document to be inserted
already has an _id property, that _id will be used
instead of an automatically generated one.

I updated tests, browser builds and the README.
Move License from README to LICENSE.md. Fixes #183
Rename LICENSE.md to LICENSE
Merge pull request #184 from sbruchmann/fix/183

Move License from README to LICENSE.md. Fixes #183
Fix for strange Date behavior in Node Webkit
Merge branch 'master' of github.com:mondora/nedb into mondora-master
One more test and recreate browser version
Bump version
Merge branch 'master' of github.com:szwacz/nedb into szwacz-master
Added a test
Bump version
fixed merge conflict